GameTunnel.Com has published a year end roundup of the top 10 indie games from 2005. These games may not have state-of-the-art graphics, but they have solid, enjoyable gameplay.
My personal favorite here is Ska Software’s Zombie Smashers X. Ska Software has a bunch of cool cheap, downloadable games, none of which I can link to here because the dumbasses did their entire website in Flash. If vampire ninjas or zombies vs. pterodactyls are your thing (and really, why wouldn’t they be), then try to put that aside and check out the site.