These are tools that one should not be doing .net development without (in my opinion) 
most are free.  I'd like to take this opportunity to truly thank the contributors to each of these tools.  They are INCREDIBLE and I can't imagine going back to working without them.

Invaluable Tools for DotNet development

Title	Description	Purpose	Url	cost
Reflector	Lutz Roeder's tool for inspecting .net assemblies and reading the representative code	reverse engineering for the purpose of learning how something works or helping to diagnose the cause of some behavior	Reflector	free
NUnit	Automated unint testing framework	ensuring quality software and enforce continued adherance to business requirements	http://www.nunit.org/	free
CruiseControl.net	continous build server and dashboard for .net	Identifying integration issues minutes after they are generated and compile/unit test confidence for all configured projects	CruiseControl.NET	free
Fiddler	http request manipulation tool	Create explicit request contexts to test the behavior of your site at a low level	http://www.fiddlertool.com/fiddler/	free
TestDriven.net	Visual Studio plugin for Unit Testing	Integrates with NUnit to allow test exectution from within visual studio. Allows method invocation with no need for dummy form coding	http://www.testdriven.net/	free personal edition
RegEx Workbench	Regular Expression editor / evaluator	Interactive execution and testing of regular expressions	RegEx WorkBench	free
Enterprise Libraries	Framework based on Microsoft best practices	Well tested and thoughtfully designed frameworks for common business needs	Enterprise Library .NET 2.0	free

I'll try to keep this list updated and add items as I think of / discover / am told about them. Each of these represents a different aspect of enterprise development and, as a whole, reduce cost & complexity while increasing consistency and quality. Pretty sweet for FOC!

Now, if you have all of these, follow to Scott Hanselman's Ultimate developer and power tool list and have a ball.  It was his post that inspired me to officially post these.  That and the desire to consolodate my url list of downloadable tools.