When I started commuting on a bike, I bought a hand pump, a cable lock, gloves, and a gel seat cover. Since I often have to ride at night, I needed a light set. Being an older bike, it didn't have pre-drilled holes for attachments, so I didn't get anything else until I got my current bike.
After getting my Trek, I bought a couple of bottle cages, then a seatbag, and a helmet. When a friend gave me a ride during rain, my lock fell off in his van, so I bought a cheap chain lock to make do for a few days until I could get my cable lock back from him. I now keep that to lock my seat to the frame, and sometimes thread it through my back wheel and helmet straps when I lock it up somewhere. Later on, the same friend bought me a U-lock in return for a favor that I didn't think was that big a deal. About a month ago, I got a cycling computer; I don't know what I'm going to do with the information, but I've been keeping track of my mileage, average speed, and time for various trips. My commuting led me to get the latest two things: a luggage rack on the back, and I changed the knobby off-road tires that came with the bike for some road tires, both sets from Bontrager. I've had the new tires for only a few days, but they seem to be making a difference so far, making my rides faster and smoother.
For a while after getting my current bike, I continued to use the gel seat cover, but I've stopped, because I don't really need it with this one like I did the previous bike; furthermore, it seems even a little counterproductive, and I feel better off without it.
For now, about the only thing additional that I want for my bike is a set of toe clips for the pedals. A maybe is a trunk bag for the rack, though the bag I'm currently using seems to be working out fine, even if it's not specifically for cycling.