db4o is the world's leading open source object database for Java and .NET. Leverage fast native object persistence, ACID transactions, query-by-example, S.O.D.A object query API, automatic class schema evolution, small size (http://developer.db4o.com)
math lib for .NET. n-dim arrays, complex numbers, linear algebra, FFT, sorting, cells- and logical arrays as well as 3D plotting classes help developing algorithms on every platform supporting .NET. Sources from SVN, binaries: http://ilnumerics.net
a project to guarantee your access to knowledge all over the world.
Monet Objects is an easy-to-use .NET object-relational mapping library. It allows .NET developers to create applications dealing with persistent objects (relying on a relational database as MySql) without writing a single SQL query.
Northwind Starter Kit project has moved, see details in the news below.