Lots of SONAR plugins work, including some of my favorites such as CA-2A, LP-MB, Channel Tools and the percussion and vocal strips.
Samplitude supports DX, so some of the Sonitus plugs work, but unfortunately not the compressor. The multi-band does, though, as well as the equalizer.
Addictive Drums, Studio Instruments, Z3ta+, Dimension Pro, Perfect Space, Rapture - all work.
What doesn't work: Adaptive Limiter, the Bluetubes collection, Lounge Lizard, R-Mix.