Here's a list of some of the features...
- Open source, LGPL licensed
- Write C#, ASP.NET, ADO.NET, XML, HTML code
- Forms designer for C#, VB.NET and Boo
- Code completion for C#, VB.NET and Boo (including Ctrl+Space support)
- Integrated NUnit support plus code coverage (NCover)
- Multi-framework support (.NET 1.1 and 2.0, Mono, Compact Framework)
- XML Editing (source and tree view) with XPath search
- Subversion integration
- Code template support
- Easily extensible with external tools
- Re-host SharpDevelop with SDA
- Easily extensible with Plug-Ins
If your looking for a Free alternative to Visual Studio or even a Windows Mono IDE this is a very good alternative.