I have been fascinated with keys since I was little. When I was in grade school I had one of those little diaries that had a lock and key. I remember the old cans of meat (typically canned ham or sardines) that had little keys you would then wind the metal seal around to open the can. I think I might have had a pair of roller skates with a key at one point as well. Somewhere I had found an old skeleton style key, which I wore for a long time as a necklace. In high school I was fascinated with escapology and got some handcuffs to practice slipping out of (and learning how to pick those locks). I have always collected 'old' keys, or found keys or really any key I can get my hands on.
I frequently see the call for a key as a focus for magical workings. Keys strongly represent opening doors, so are great to use not only for bringing new opportunities into our lives, but also for opening up places inside our selves that we might have locked away. On the flip side, keys are used to lock things up, so are good for security work or for keeping secrets. One of the first keys many of us own is a house key, so keys can represent the home and family. Keys are also used to secure the house when we are away, so they can be associated with travel and returning home safe (and guarding the house while you are away).
One of the great thing about keys is that there are so many of them. Not only more modern keys, but keys for different purposes. Not only are there many different types and looks, but because they are built to hook onto a ring (or be tied to a belt for older keys), they lend themselves well to decoration. Keys can be painted (with regular paint or nail polish), decorated with charms or ribbons, written on, etched or added to another tool as a charm. They can be used as a focus for workings, used to etch symbols on candles, hidden away or used as a worry stone.
As you build a collection of keys for different purposes, consider keeping them all in one place when not in use. I used to have an old key ring, a large ring about 4 inches in diameter that could be pulled open and keys put on it (like you sometimes see in old movies for keeping jail keys on). You could also keep them in a decorative box or bowl.
I am going to talk a bit about different types of keys and ways you might use them:
Key blanks- When you go to a store that makes keys, you can buy key blanks and not get them cut to fit a particular lock. Often they have a large variety of designs. Key blanks are great for general workings, or for one where you don't have the specific key you might need. If you wanted to have a key to remove obstacles in your normal daily life, a blank key would be great for this. If you wanted to work through some past issues you had when you lived in a particular house, but you no longer had a key for that house, you could get a blank key and inscribe or paint the address of the house on it (or some other important naming feature), and that key would then become a key to that house that you could use to work with.
House keys- Most of us have a house key. There is no reason not to bless your regular house key as an extra layer of protection. But many of us not only have a key to our current house, but we may have keys to other people's houses (family or friend), our work place, or places we used to live. Of course you can use other people's keys to bless them and their household, but these keys can also be used to work on the relationship you have with those people or as a touchstone for communication with them. Work keys can be used to open doors for promotion or toward a job you desire. They can also be used to lock away bad feelings between you and coworkers (or bosses or employees). Old house keys to places you no longer live can be used in journey work to step into the time period where you lived at that house, to connect with people you were in contact then that you might have lost touch with (or who passed on) or to connect with the location where the house is (especially useful if you have moved far away).
Car keys- The strongest association we have with car keys is with travel. But cars also represent freedom and independence (trust me, for at least half the year I have no car during the day). When we are young and get our first car, it is often the most dramatic way in which we experience our separation from our parents...it is the first way in which we can really go our own way. More than any other key, a car key represents choices: when we get in the car, we have many choices in where to go and which road to take to get there, so a car key is a great key for working on life path problems. As most of us rely upon our car to get to and from work, it is also a good key to use for job related things (if you don't have a key to your job, or if you want a job that you don't have). The car is also the main way most of us bring things into our house (such as groceries, clothing, or really anything that we have to drive to collect), so it can be used to work on bringing abundance into our lives (or for letting go of the things we no longer need).
Door keys- When we moved into the apartment we are in now, there was a little key on top of every door frame. Really not more than a little twist of metal with a slightly flattened end, they can be stuck into the hole on the interior rooms to unlock them if they are locked from the other side. This might not be a very common key, but I think it is a wonderful key to have around. Having been married for just past 15 years now (with a teenage son just about to enter high school), I can attest that communication between people living together is very important to a smoothly running (and happy) house. Though the doors within the house give us the ability to block each other off, the door key gives us a tool with which to open up to each other.
Skeleton keys- These are the old fashioned keys (technically they are a subset of older keys, but if you say skeleton key, most everyone thinks about an old style key). You know the type, they have a 'tooth' at the tip, and a round loop at the end (for attaching the key to things and to give you something to grip as you turn it). They can be simple or ornate. It has become quite popular lately to make charms or talismans out of this type of key, and when most people think of keys for magical work, this is the type of key they think of. They are not the most secure type of key, in fact, many skeleton keys were a type of master key (or lock pick tool), in that they weren't made to open a particular lock but to be able to bypass the locking mechanism and open it anyways. I think this makes them ideal for overcoming obstacles or blocks in your path, especially if you aren't sure of their exact nature.
Handcuff keys- It is pretty easy to come by these today. Especially around Halloween, many stores sell handcuffs as costume props, and they come with a key. The thing that I think makes a handcuff key unique is that it is designed to hold people and not things. These keys (and the cuffs themselves) can be used to bind people who would seek to harm you. You can also use the key to work towards overcoming any limitations you feel are holding you back.
Box keys- I am kind of lumping together any key here that locks a box: safety deposit keys, locker keys, lock box keys. These keys are the ones that guard our things, especially the things we hold most dear. They are also strongly associated with money, so good for working towards protecting our monetary assets. Most of the time, you use these types of keys to lock things away behind extra protection from thieves, so they are good to use to protect you from thievery.
Diary keys- There are still a lot of diaries that come with tiny keys. I love these keys for protecting thoughts, secrets, wishes or anything else that is personal and private and deals with the mind. It is common to build dreams and hopes in a diary, so they are also good for working towards the desires of the heart (both love and other things we yearn for...emotional desires).
Key cards- These are more of a new thing, and won't always fit with the other keys you may work with. But a lot of hotels are using them now, and because of their temporary nature (I have yet to be asked to return they key card after my stay was finished), they are good for working towards short term goals, especially those involving travel. Some attractions (theme parks or other things you buy admission tickets for) now use plastic cards instead of paper tickets, although either would work as a 'key' to the attraction or event. These can be used like keys to old houses: they can connect us to our memories of the past, or to the essence of the event. They can also be used to bring that energy into our future (if it is a place or experience you would like to revisit).