For .NET development, VS2008 is the best but if you want to check for another best IDE, Eclipse probably the best after VS if you are rating it among the IDEs, ofcourse you cant do .NET development in Eclipse though
The other great thing about SharpDevelop is the ability to translate solutions between the two big managed .NET languages VB.NET and C#. I believe it doesn't work for "websites" but it does for web application projects.
If you are looking to try Java, I believe NetBeans is a very, very good IDE. However, for .NET, sure there are alternative IDEs but I don't think it makes much sense to use them unless you are developing on an Open Source platform, in which case SharpDevelop is a good choice and is reasonably mature.
There's MonoDevelop, which I occasionally use when I want to do some light C# coding when in Linux. It's nothing close to VS.Net, but it works for small projects. I really don't think most of the alternatives people have listed come anywhere close to VS.Net.
I still like Source Insight a lot, but I'm hesitant to recommend it anymore as I'm not sure anybody's still maintaining it. They released a very minor update back in March but haven't had a major release in years. And there seems to be no web community presence. It's a shame because I still like its auto-completion-friendly file open and symbol browsing panels (as well as syntax formatting) better than anything else I've ever used.
Try out this one: "Pao" at http://pao-ide.info . It's still in development and not up to production use but it's quite unique in features. Basically, all the language constructs, such as assemblies, types, members, statements, expressions are treated as objects and are associated with rich operation options. You can enjoy features usually seen in graphical editors, such as multiple selection, multiple copy-paste, tagging, batch operation and very powerful search capability. It takes some getting used to but eventually might increase productivity. Right now, it only supports form applications though.
Eclipse has several C# plugins, the best I've found to date is Emonic . You can target .NET or Mono with the help of it. Both Eclipse and Emonic are open source.