LastFmLib.net lets you access all Web services provided by Last.fm. This also includes an implementation of the Audioscrobbler 1.2.1 submissions protocol and radio api. Also, LastFmLib.net provides a set of useful classes, e.g for caching requests and pictures and for accessing simple request easily via the LastFmClient class. The library should be fully Mono compatible (the test with MoMa was positive).
Information regarding Project Releases and Project Resources. Note that the information here is a quote from Freecode.com page, and the downloads themselves may not be hosted on OSDN.