DSM is the closest thing I know of to MSpectralDynamics. The two are equivalent only conceptually, though. (Also, DSM is more expensive at $329.)
To picture what MSpectralDynamics does, imagine a dynamic equalizer with hundreds of bands, configured to describe a specified overall spectral shape. Great for mastering. But it doesn't end there...
Now imagine that you can route a sidechain signal to the analyzer portion to spectrally compress one signal based on another, but only effect those frequencies that are contained in the sidechain. That lets you carve a spectral hole in one track or bus that perfectly fits the needs of another track or bus. You now have the ultimate anti-masking tool.
If you a) don't do your own mastering, and/or b) you have no idea what spectral masking is, then save your money, just get Sausage Fattener and have fun. (Just kidding, peter434. Spend the money on MDynamicEQ instead, next time it goes on sale for $43.)