Also there are basically two type of plugins (not only literally TWO, but I like to put it like this): the generators and the processors. Being a self producer most interested on record my own stuff I get specially excited with the generators that offers me a new gamma of options in sounds. Lately I have been very focused on the ones that try to mimic traditional instruments and old electronic stuff.
My latest free acquisitions were:
Ixox Flute (transverse flute)
Milestone (trumpet)
DVS Saxophone (saxophone)
DSK Brass (orchestra brass section)
DSK RhodeZ (Fender Rhodes)
Minimogue (minimoog)
If you have interest on non-free stuff I would take a look also in:
Real Guitar (acoustic guitar)
Real LPC (Gibson Les Paul)
Real Strat (Fender Stratocaster)
GSI VB3 (Hammond B3)
NI FM8 (FM Generator)
As someone has well put before you have to try and judge for yourself though. Specially at the sampled instruments ballpark you cannot expect miracles from a free VST, but with the proper tweaks and a couple tricks you can make them sound surprisingly convincent for most of people. I achieved excellent results with the mentioned tools above. Additionaly, an equivalent 'pro' tool may cost hundreds of dollars, push you into a long learning curve and at the end of the day do not make too much difference in the final product.
Also, even with the best tool you may not reach
that final quality because the plugin won't do everything for you. The other day I watched a demo video of a tool called 'The Saxophones' from a company known as 'Sample Modeling' (
Sample Modeling | SWAM platform). Really awesome (with a capital 'A') tool that supposed to mimic a sax in the minimal details and with an impressive quality able to fool even an experienced saxophonist. Well, the
tool can help you do that but will
you able to extract all its power? In the video there was a keyboard player playing like crazy and doing spectacular things. Cool. Nice. If I had this tool I wouldn't be able to enjoy 10% of this software power though. First, I don't have any keyboard. Second I play keyboard like a hammer, don't have the proper sensibility or dexterity. Third, I wouldn't know how to play a musical track as a saxophonist does. Fourth... well, you got the point.
I think that sometimes less is more.
Of course be wise with my words -- each case is one case.
Good luck!