For .NET

Resharper (free-to-try, €131)

Resharper is an extension for, in general, intellisense. It’s very powerfull in different ways. E.g. it checks your code for compiling issues on-the-fly and giving possible options to resolve it automatically.

download:
http://www.jetbrains.com/resharper/download/index.html

BugAid (free-to-try, $49)

BugAid is an extension for the variable watcher in debug-mode. Convince yourself with this video http://youtu.be/0GuAq76-D-s.

download:
http://www.bugaidsoftware.com/

For SharePoint

CKS: Development Tools Edition (free)

This project extends the Visual Studio 2010 SharePoint project system with advanced templates and tools.  Using these extensions you will be able to find relevant information from your SharePoint environments without leaving Visual Studio.

download:
http://cksdev.codeplex.com/

Visual Studio 2010 SharePoint Power Tools (free)

Also extends Visual Studio 2010 with templates and tools, especially focussed on sandboxed solutions.

download:
http://visualstudiogallery.msdn.microsoft.com/8e602a8c-6714-4549-9e95-f3700344b0d9