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…