I wrote ages ago about developing a tool to make nunit tests run in multiple threads part1 and part2. That was pretty good however it hit a slight issue in that if the tests weren't good about the use of global statics, then things could get very messy.
Last week I wrote about my desire for an nunit runner that could execute tests in parallel batches. Specifically selenium tests that were using the nunit framework. It turned out to be pretty easy to implement.
In my new team there is considerable use of Nunit for unit tests, which has been further adopted as a convenient mechanism for all tests. Including a set of selenium based UI tests.