In the vein of my Ultimate Tools List, here's my list of Ultimate Visual Studio.NET Add-Ins.  This list is by no means exhaustive, but I loves me some Add-Ins, so here's the ones that make my life better.

• CodeRush - Of course. It's the bomb, enough said.
• Peter Blum's ADME - ASP.NET Design Mode Extender (“ADME”) helps custom controls to provide a better design mode interface. This supports his Validation Controls, which rock, but also controls that you might write that need richer Design Mode Support.
• CodeSmith Explorer - Generate CodeSmith code and templates directly from VS.NET
• CopySourceAsHtml - Better than a Macro, this Add-In puts syntax-highlighed HTML on your clipboard. Now it supports "Embedded Styles" for use in BlogJet and other tools.
• GhostDoc - Here's an Add-In I'd overlooked previously, now in it's 1.2 version.  GhostDoc attempts to generate C# documentation that can be gleaned from the name and type of methods and properties. Roland Weigelt has big plans for version 1.30 that will include customizable text and rules. One to watch, and while it sometimes guesses wrong, it's a completely unique Add-In worth your download.
• devMetrics - devMetrics is a free tool for measuring various attributes of your C# code so that you can accurately assess your product for quality and maintainability. I use it to measure cyclomatic complexity and abuse people during code reviews.
• QuickCode.NET - This has been largely supplanted by CodeRush, but if you don't want to pay as much, this cool tool gives you phrase expansion and a surprising number of code expansions available on their user forum.
• Mindreef SOAPscope - The original. The glory forever, this is more than an Add-In, it's a complete XML Web Services design studio. It's a bargain and works even better when setup in a workgroup. It keeps a database of all Web Services traffic, but it's more than a sniffer. It also analyzes data for WS-I compliance and allows for record and replay of messages. "It's Tivo for Web Services!"
• NUnit Addin, now TestDriven.NET - If you're serious about TDD, stop fooling with NUnitGui and Attach Process and start using TestDriven.NET. It's a simple as Right-Click -> Test With -> Debugger.
• pinvoke.net - Adam Nathan continues to innovate with an add-in that lets you "Insert PInvoke Signature" from the VS.NET Editor by communiating with a server-side repository with best-practice signatures to make calling unmanaged code a breeze. Also, be sure to visit the PInvoke.NET Wiki.
• Reflector as an AddIn - A joint effort, run Lutz's unbelievable decompiler/explorer with Jamie's Add-In support. (There's a number of other slick, but alpha-quality addins at that link as well, including FxCop as an AddIn.)
• Regions AddIn - Finally, something useful from CodeProject, this add-in helps organize your code with a simple Right-Click -> Add To New Region and Right-Click -> Add To Existing Region. You'll wonder how you lived without it!
• Unleash it! - The great ASP.NET deployment tool with the unfortunate name. Formerly known as WebDeploy, this Add-In lets you deploy your ASP.NET application using whatever it takes. Now with plugin support!
• WS Contract-First - Christian Weyer leads the pack with custom Web Service code generation, and coming soon, generation of WSDL itself from Message-based XSD. How's that for SOA and contract-first development?
• VSCmdShell - Open a Command Prompt within a Visual Studio.NET 2003 Docked Toolbox Window!
• CommentReflower - Really detail-oriented? This tool reformats your code comments to your specifications.
• OnlineSearch - Search the Internet and Google directly from VS.NET!