Open Source Alternatives to Reflector?

Just to ask if anyone knows of an open source alternative to RedGate's Reflector? I'm interested in checking out how a tool similar to Reflector actually works.

Note, if you know of a free but not open source alternative to Reflector, you can answer the following related question:

• Something Better than .NET Reflector?

Summary - Updated 11th May 2011

A quick round-up of the various open source projects and tools that have been suggested:

1. Common Compiler Infrastructure (CCI)
2. Mono Cecil
3. ILSpy
4. dnSpy (fork of ILSpy, project appears more active than original)
5. Dotnet IL Editor (DILE)
6. IL.View
7. Monoflector (no longer active as of April 2011)

The following resources may also be of interest:

• TypeView.cs
• Jason Haley's notes on disassembling .NET
• Adrian Bank's recent blog post summarising a number of Reflector alternatives, including several options not mentioned below.
• Mark Lichtenberg's detailed blog post comparing several of the open source alternatives (DILE, ILSpy and Mono Cecil using MonoDevelop) to Reflector.

2 options I know of.

• CCI
• Mono Cecil

These wont give you C# though.