Hello. I've a project in .NET Framework 3.5 and as i can see nUNIT does not support it yet. What unit testing framework would you recommend for my needs?
NUnit works perfectly well in .NET 3.5 - I've been using it (with ReSharper running the tests) since the betas of VS2008.
As Jon said NUnit works fine with 3.5.
There are a few things you need to keep in mind:
- There are some (very few) obscure breaking changes when you upgrade to later versions of the framework, usually due to the use of deprecated APIs or non-standard methods. In my experience fixing these kind of breaking changes is usually fairly simple once you catch where its coming from. 
- If you are getting a particular bug with NUnit, isolate it in a tiny test suite. When hunting for bugs I usually assume its my fault and usually I am right. 
- The source code for nunit is out there you can compile and debug into issues. Testing frameworks, in general, are fairly straight forward and easy to follow. 
- There are plenty of other unit testing frameworks. The more opinionated XUnit framework is gaining popularity. Personally, I like fluent assertions but its a personal taste thing that can also be achieved with NUnit. 
- You can always fall back to Microsoft's built in unit testing framework, which ships with VS. 
Some other unit testing tools for .NET 3.5 would be:
- MS Test - the built-in unit testing framework in Visual Studio. It's not nearly as bad as its reputation.
- xUnit.NET, which has a very nice API.
Personally, I use MSTest since it is built into Visual Studio. It just makes it easy since you don't have to install anything in order to use it.
I ran into this issue as well, and was able to work around it, in NUnit 2.5.
In the NUnit GUI, turn on Visual Studio support under Tools > Settings...
Now choose File > Open Project... Change the file filter from "Projects & Assemblies" to "Solutions", and load your solution file. You should be able to run all of your tests.
Now choose File > Save As... and save the current setup as an NUnit project. You can now go into Project > Edit... and remove non-test assemblies or make any other changes you need.