Unity, http://unity.codeplex.com, Ms-PL license
Developed by Microsoft P&P.
Spring.Net, http://www.springframework.net/, Apache 2.0 License
Port from the famous Spring framework.
Castle, http://www.castleproject.org/, Apache 2.0 License
The first .Net IoC Container?
StructureMap, http://structuremap.sourceforge.net/, Apache 2.0 License
Looks good.
The fifth?
[updated on Sep 14th]
Yes, just know the fifth,
Ninject: http://code.google.com/p/ninject/ Apache 2.0 License. No comment yet…