There are two main kinds of paint can openers that are available. There are the ones with the plastic handle, like this:
and there are the all metal ones with a loop at one end, and the opener at the other end. In my experience the plastic-handled ones cost about $1.49, and the metal ones cost about $0.49. Price aside, there is one good reason to prefer the metal ones to the plastic ones, and that is that the loop end is a bottle opener, and as James says, "Nothing goes together better than painting and beer."