In addition to the great ones already listed on the link and in the comments here, I'd like to list 2 of my favorites:

* Simple.Data: https://github.com/markrendle/Simple.Data , the smallest and simplest ORM you'll ever use.

* Hyperletter: https://github.com/Jiddler/Hyperletter , a very easy way to do inter-process communication. Great for writing distributed software over http, but can be just as useful locally.