Smart Smoother High Quality is a good spatial denoiser for VirtualDub, which tries to keep edges sharp.
For AVISynth, there is fft3dfilter, a spatio-temporal denoiser with loads of options that I haven't really worked out how to use. And the old Pixiedust is good but slow. There's also a new function you can find at Doom9 called RemoveNoiseMC which seems to give good results.