What are the most useful, free .NET libraries?

Question

Please ignore. DUPLICATE to http://stackoverflow.com/questions/662956/most-useful-free-net-libraries

There's a similar question on SO for Java. I didn't see one for .NET. What are the best can't-live-without free (gratis and/or libre) .NET libraries?

This is a poll, so please:

• Post only one library per answer
• to avoid duplicate answers, before posting check if the library has been mentioned already.

If there's good interest and response, I'll summarize.

Answer 1

Actually, this question does list free libraries for .net. I highly recommend checking it out, lots of good stuff in there.

Answer 2

My bid is The C5 Generic Collection Library

From the page:

C5 is a library of generic collection classes for C# and other CLI languages and works with Microsoft .Net version 2.0 and later, and Mono version 1.2 and later.

C5 provides functionality and data structures not provided by the standard .Net System.Collections.Generic namespace, such as persistent tree data structures, heap based priority queues, hash indexed array lists and linked lists, and events on collection changes. Also, it is more comprehensive than collection class libraries on other similar platforms, such as Java. Unlike many other collection class libraries, C5 is designed with a strict policy of supporting "code to interface not implementation".

Answer 3

HtmlAgilityPack - Html parser allows you to easily read/write the DOM. Lifesaver.

Answer 4

NUnit. Unit testing done right; massively useful.