A long time ago, I started knitting a sweater for my bicycle. Today, because I had the bike in the house doing some maintenance, I decided to put the sweater on the bike. I don't care much for the look of it, and I think I am going to do something else, but I will leave it on for a while to see if it grows on me.
Out of curiosity I went on Ravelry to see what other people might be doing for their bikes and of course I found a lot of projects. All images link back to the profile pages of their creators on Ravelry and I don't think they will work if you aren't logged in.
More Bike Cozies
Seat Cozies
Bike Racks
Locks
Attempted Helmet Improvements
Baskets and Such
Other Cute Stuff