Visual Studio is really unmatched, with all of the things that it can do. I still use Resharper and a few other plugins to tweak things out just the way I like them. You could do all this with a customized vim or emacs or whatever editor, but the learning curve is much lower. And while visual UI editors tend to be a trainwreck, the VS ones are about the best.
RedGate makes a really nice .net performance and memory profiler. One of the things I haven't seen elsewhere that it does is being able to drill through any ORM libraries to see the SQL that is being generated and executed against your databases.
LinqPad is a REPL, but it does intellisense, Nuget integration, rich output options.
It's generally just a little bit more finish and usability than OSS options