I think Xbox Live Arcade's pricing has redefined the price point for download-sized games on the PC, as well. For example:
$5: Chime, Doom, Geometry Wars, Sonic, various arcade ports. Arcadey, pick-up-and-play gameplay, good for short bursts.
$10: 'Splosion Man, Alien Hominid HD, Ikaruga, Rez HD, The Maw. Games that have a retro look, but gameplay that is deep and lasts a few hours, at least.
$15: Banjo-Kazooie, Castle Crashers, Serious Sam HD, Shadow Complex. Deeper games running on fancier engines, often produced by big-budget studios.
$20: Penny Arcade, Sam & Max. Pretty much reserved for episodic content.
I like the pay-what-you-want model, especially when a PayPal or other donation link is available aside from the download link itself.
RunMan: Race Around The World gives you the game for free, if you want it.
Rifftrax, while not a game, has a donation link set up for people who downloaded a Rifftrax'd movie over bittorrent or watched a streaming video.