If you're like me you were super disappointed when Red Gate reneged on their promise to keep .Net Reflector "free forever". While I don't use Reflector every day, there are numerous times where I've needed to drill into a dll to see how something is accomplished or find some functionality I need.
Fortunately, there are new alternatives appearing for us penny pinchers. Hopefully Red Gate will get the message.
ILSpy - ILSpy is a free, open source replacement for .Net Reflector. Look and feel are modeled on Reflector.
JustDecompile (h/t Assia) - Telerik's foray into the market. Currently in beta but promises regular updates and an integrated auto-updater.
dotPeek - By JetBrains.