Visual Studio does its job well. Its still the best .NET IDE out there. Unfortunately there's a lot of day-to-day stuff that it just doesn't do. This is where third party tools come in.

Here are my favorites. The great thing about these tools is that they have all been around for a while, and have tons of support and documentation, so there's very little chance of them being abandoned. Best of all, they're all free.

WinMerge

WinMerge is a really easy to use Open Source Diff tool, that can be used to compare both specific files and directories (which is great for merging project branches, hence the name). It comes bundled with a few useful filters (such as "exclude source control files"), which are easily modifed for your own purposes.

Tortoise SVN

Tortoise SVN is a front-end for Subversion, the installer includes both Tortoise and SVN. Mercurial is also becoming quite popular as Distributed Version Control Systems are taking off, but most companies I know of still use SVN. There's a Tortoise GUI for Mercurial as well.

Nunit

Nunit is an open source unit testing framework for .NET. It works really well, and is quite easy to use.

Test Driven .NET

Test Driven.NET allows you to run a specific unit test within Visual Studio, and even allows you to "run with debugger" which is very useful. There's a free personal version available.

Notepad++

Visual Studio is a great IDE, but is definitely overkill for simple tasks like editing an XML configuration file. Notepad++ is very lightweight replacement but has a few important features like syntax highlighting and support for plugins. The Zen Coding plugin is absolutely essential if you're a web developer, it makes writing large amounts of HTML very quick and easy.