Miranda-IM - a small, multi-protocol (AIM, MSN, ICQ, IRC, and Yahoo!) IM client. I usually don't like/need the bells and whistles of most clients, and this perfectly suits my needs (the download's only about 700kb, too). Like most other things on this list, it's extensible.
Foobar2000 - Music player. Much lighter than iTunes, but not a very good interface. I love the tabbed playlists, though. Yis. Useful when I just need a light player.
AbiWord - Microsoft compatable word processor. All I really need when I write are a) wrapped text, b) a word count, and c) a spellchecker, because my spelling suck smelly monkey bottoms. That's not all the program can do, but it's a far sight less bloated and resource eating than Word.
So, that's all I have right now. I like them a lot, but I know they're not for everyone.