Test Driven Development
Books

Lessons Learned in Software Testing by Cem Kaner, James Bach, and Bret Pettichord.
Pragmatic Unit Testing in C# by the Pragmatic Programmers Andy Hunt and Dave Thomas, the preview chapters (one, two) look good.
Managing the Testing Process by Rex Black – highly recommended by John Robbins.
Test Driven Development: A Practical Guide by Dave Astels, we've got this at work and it is very good although the examples are in Java.
Test Driven Development: By Example by Kent Beck. My comment: can we please move away from the stupid money example.
Test-Driven Development in Microsoft® .NET by James Newkirk, a developer on the NUnit project. Awesome book. (get all the code from the TDD workspace on GotDotNet)

Software

csUnit - open source testing framework
NUnit - open source testing framework
mbUnit - unit testing similar to NUnit, from a Microsoft employee

.TEST - commercial automated unit testing software
HarnessIt - commercial testing framework
Unite.NET - commercial unit and integration test software
X-Unity - commercial testing framework

NUnitAddin - Visual Studio add-in, currently has command-line interface (I hear Jamie Cansdale is working on a GUI with red/green bars)
NUnitASP - test web sites by interacting programmatically with the controls on the web page. Now supports authentication as well as several other new features! Also see my NUnitAsp Tips and Best Practices.
NUnitForms - allows you to test Windows Forms in a manner similar to NUnitAsp (you get to interact with the form elements programmatically). It even includes a recorder application!
RUnit - runs NUnit test harnesses in ASP.NET.
TestRunner for NUnit - an NUnit add-in for Visual Studio .NET. See blog post.
VSNUnit - an NUnit add-in for Visual Studio .NET. See blog post.

Mock Objects

DotNetMock
EasyMock.NET
MockObjects
NMock
POCMock

Web sites / columns

Applying Patterns to Software Testing - Brian Marick
Bret Pettichord's Publications on testing
Brian Marick's writings on software testing
Cem Kaner's articles on testing
James Bach's articles
ObjectMentor's Test Driven Development articles
TestDriven.com web site, general purpose portal that includes various languages, testing tools, and articles.
Test Driven Development Column Dave Astels
Test Driven Development Yahoo! group
Pages on the c2 wiki:

TestDrivenDevelopment
TestingCategory
UnitTest - check out all the links at bottom
Blogs

Benjamin Mitchell has an excellent XP and Agile category including lots of TDD
Brian Marick's blog is all about testing
James Newkirk, lead developer on NUnit and now Microsoft employee, has an excellent TDD blog.
Jay Kimble has a TDD category
Jens Winter has a TDD category
Jonathan Cogley has a TDD category
Jonathan de Halleux - the creator of mbUnit
Roy Osherove has a Test Driven category
Siva Rama Krishna has a blog devoted to software testing
Steve Eichert has a TDD category
See my Test Driven Development category

Unit Testing articles

Adventures in C#: Using NUnit - Ron Jeffries
Advanced Unit Testing: Parts 1, 2, 3, 4, 5 - Marc Clifton
An Initial Investigation of Test Driven Development in Industry - Laurie Williams and Boby George
Best Practices: Test Driven Development
Charles' Six Rules of Unit Testing - Charles Miller
Contractual Test Driven Development: Combining Test Driven Development and Design By Contract - Dave Chaplin
csUnit Tutorials - a group of related tutorials on csUnit, including one on how to port from NUnit
Developing Automated Tests using NUnit2 with VB.NET - Dave Chaplin
Effective Unit Testing - Tim Burns
Endo-Testing: Unit Testing with Mock Objects - Tim Mackinnon, Steve Freeman, Philip C