JetBrains has thrown their hat into the .NET decompiler ring with the recent release of dotPeek. I've been using it for a couple of days now and have found it to be a fully-featured competitor to the standalone .NET Reflector by Red Gate Software.
The standalone version of .NET Reflector is priced at $70, while dotPeek is free. I wonder if this is going to change anytime soon. .NET Reflector also has version that adds Visual Studio integration to the basic utility (available for $130) and a Professional edition that adds VS integration and some very useful debugging features (available for $190). Sadly, I have not been able to convince my program manager to buy any licenses for our team.
dotPeek doesn't do anything other than decompile assemblies, but it does that well. This is good news, since I have really missed having a decompiler available in my daily work.