SciYro, nope .net isn't a language it is a framework, and has been ported to work with C, Java as well as Visual Basic and ASP [ VBScript ]
Mono has limited support for current MS VB and VBS versions.
tkvs,there isn't an open source framework that is compatable with .net, but there are open source frameworks for web development that are comparable, and newer releases of most distros include them as options. [ Ruby On Rails and AJAX ]
There is also some open source projects that are adding .net support to open source tools. look for development packages in your distro repositories with "sharp" in the package name, they add the .net framework support.
I would check the dev pages of the php site [
http://php.net ] to see what the status is of any comparable framwork project. Not to forget, the Zend studio and tool set that are meant to improve php productivity.