Background: about a month ago I bought a STK500 and started working on a project that's coming along nicely. A buddy of mine has been working with me on it, and now that it's to the point where we have two prototypes it would be nice to get a second programming device rather than having to use the same STK500.
My question is what is the best programmer to get optimizing for cost and ease of use for my buddy who isn't much of a hardware guy. We'd like to program our prototypes using avrdude (if that makes a difference). The programmer doesn't have to set fuses or anything else that only needs to be done once, since we can do that with the STK500. He's going to use his MacBook, so USB compatibility would be nice if it costs around the same as a USB->serial adapter.
I have an assortment of misc AVRs already if they can be easily converted to a USB programmer.