I don't use this myself, but a few of my friends do and they say it's great.
The <a href="http://www.udpsoft.com/eye2/index.html">All Seeing Eye</a>.
You can download a 21 day free trial, and if you like it you upgrade to the full version with more benefits. It doesn't just work for Half-Life though, so if you play a lot of online games, I would recommend trying it.