Microsoft opens .NET source code. Ok, it would have been even better if it had been an open source license right away (and with GPL they could have even protected themselves), but hey, this variant is not uncool either - co-licensing of patents for the reference sources is already a good move. And Mono has gained a bit more security.
dotnet
JSIL - .NET to JavaScript compiler. No idea if I already had this, but it's one of those crazy projects you can safely link twice. The actual .NET IL code is translated - so you can run programs in the browser that were not really intended for it. For example, part of XNA is supposed to run, so you can run games in the browser that are actually for the XBox.
BOO - Getting Started. After a long time, I took another look at Boo and I like what's happening there. Still very strongly based on Python in terms of syntax, but semantically quite different - for example, real macros in various variations, so that there is really an extended syntax. Or also the optionally available Duck Typing - a variant that I particularly like, because Boo combines static typing, type inference and Duck Typing in a way that really keeps all options open for the programmer. Then there's the good integration with .Net and C# assemblies. I think I need to play around with it a bit more, because as a prototyping language it could be quite practical - and at the moment I tend to use Mono more as a runtime, primarily because of the JIT implementations available there (and with Mono also AOT compilation, where it is needed).
NancyFx/Nancy. Nancy is already much closer to Sinatra or Scalatra, but is actually based on C# - but maybe you can also just write modules in F# there, the interoperability of the two languages is very high (comparable to Scala and Java).
Bistro. This is now a web framework for F#, inspired by Django (among others). Unfortunately not as lean as Scalatra or other micro-frameworks, but perhaps still worth a look.
RaptorDB sounds quite interesting, a classic Key/Value-Store for .NET that is based on MurMurHash and is especially designed for performance and storage stability. Could be quite useful for one purpose or another, especially since it also works cleanly with threading.
IgniteInteractiveStudio/SLSharp. Net - Write GLSL Shaders in C#, the IL code is then automatically loaded onto the GPU. High-Performance-Computing anyone?
How I learned to stop worrying and write my own ORM. A bit of background information on why Dapper was developed and which use cases it solves - it is essentially used where direct SQL access was previously "tunneled" over Linq for performance reasons, because Linq2Sql is inefficient there.
Dapper-dot-net - Simple SQL object mapper for SQL Server. Could potentially be quite interesting at work. C# also offers Linq, but according to their measurements, Dapper seems to be significantly optimized for performance.