I found amazingly few references to people having problem with the NUnit test runners. We came to a point where the the NUnit test runners consumed over one gigabyte of memory and the same tests run from Resharpers test runner from inside Visual Studio would only consume about 250 megabyte of memory. I also found that the memory was not released between assemblies if you run a NUnit project file with multiple assemblies.
The problem was that a reference to each TestFixture decorated class was kept in the result even if it was unnecessary. Or to be more specific, the Fixture part of the result released it's reference to the tested Fixture class after TestFixtureTearDown, but the TestMethod in the result kept a reference to the very same Fixture...To find out about this I used a nice memory profiler, SciTech's .NET Memory Profiler
I posted a patch to the NUnit project on SourceForge. With this patch the nunit-console test runner use about the same amount of memory as the Resharper one, and I suppose this would be the case for the GUI one too, since this was a change in the core.