OK, this one is a “no-brainer”. The newest version of Sampletank Free (2.5) is a “full-featured” version of Sampletank. It comes with 500MB of samples for free (don’t forget to download them; they’re separate), but unlike previous versions, can also be expanded just by purchasing the sample sets available from the “Powered by SampleTank” range and any of the “Expansion Tank 2” sound sets, as well as importing your own WAV, AIFF, SDII, AKAI S-1000/3000, and Samplecell files.
It has all of the features of the full fledged product, just not the sample sets (which can be purchased at a later time if you wish).
Again, for free? A no-brainer must-have. Seriously.