Scott Watermasysk has a post a thought provoking read about his top five favourite developer API's.
From these API's I have worked with:
- RSS.NET which I used to cobble together a rss feed from our build machine. I combined this with a X10 light configuration to provide a visible tactile build status.
- Genghis which is just awesome for adding those bits to applications that MS didn't have time for.
I'd also rate:
- Log4NET - who doesn't need logging
- FileHelpers - because reading files is what it's all about in the end