NeuronDotNet It is not as broad in its scope as AForge, but its exclusive focus on NN makes it maybe more readily accessible, and possibly more featured in this area.
Update: (December 2012)
NeuronDotNet's old repository at http://neurondotnet.freehostia.com is defunct.
The most recent source code, Version 3.0 is available on sourceforge, but this project is effectively inactive. Apparently, its original developer, and sole contributor, Vijeth Dinesha, has stopped updating and improving this code base. Thank you, Vijeth! Maybe someone will, some day, pick-up from where you left. Indeed, NeuronDotNet is a relatively mature framework and its simplicity and single focus make it attractive, compared with other frameworks where Neural Nets are just one feature/module therein.