Here's a list of some of the .NET libraries I use in my production / development code, organized by what I use them for:

Logging

• NLog - NLog is a free logging platform for .NET with rich log routing and management capabilities. It makes it easy to produce and manage high-quality logs for your application regardless of its size or complexity.

Unit Testing

• NUnit - The all-purpose unit testing framework for all languages in the .NET framework. It's free, robust, and powerful.
• Approval Tests - a new, behavior-driven testing methodology that uses one-time input from a human source to validate the results of your classes. Much less tedious than using lots of NUnit asserts for some types of activities. [new:7/10/2010]

HTML Parsing & Page Scraping

• HTML Agility Pack - An agile HTML parser that builds a read/write DOM and supports plain XPATH or XSLT. It is a .NET code library that allows you to parse "out of the web" HTML files. The parser is very tolerant with "real world" malformed HTML. The object model is very similar to what proposes System.Xml, but for HTML documents (or streams).

Generic REST-Handling APIs

• Hammock for REST - A service-agnostic library for querying REST APIs. It's brand-spanking new and I still haven't fully figured it out, but I've been able to run it against the SlideShare and YouTube APIs with accurate results thus far.
• RestSharp - Another service-agnostic library for consuming REST APIs. Comparable to Hammock in many ways, but currently does a better job making it easy for developers to deserialize REST XML directly into POCO classes. [new:7/10/2010]

Social Media APIs

• TweetSharp - A extensive .NET library for querying microblogging services like Twitter, Yammer, and Identi.ca. Built on top of Hammock.
• Google Data (GData) API Client for .NET - The offical GData .NET client developed by the Google team. Includes libraries for every Google service ranging from Google Apps to YouTube.
• SlideShare API Wrapper for .NET - An extremely simple wrapper library for the SlideShare API.
• FlickrNet API Library - A .NET library for accessing Flickr's API.