You should check out the website for Orion Press: http://www.fastcopyinc.com/orionpress/archives.htm - I don't recall if it's *all* gen, but at the very least it's *mostly* gen, and there's a ton of fic.

My very favorite author for McCoy-centric TOS gen is Cathy German, though she doesn't have a website (that I know of) and the only stories of hers that I've been able to find are scattered throughout the Orion Press site. Here are some:

Last Picked
The Tale the Cap Told
There Would Be Others
Trapped