I know there still some Shareware and Freeware VSTs unposted on the downloads sections, like VSamp, Crystal, Vivaldi, CesSynth, FmHeaven... and so on ... but most of them are still easily avaliable on the net, so there is no hurry.

But, what are the  imposible or hard to find missing ones?

I'll begin the list:

Sampletank v2: We have the version 1 but not the version 2, which includes some enhancements.
LM4 MKII: we have the LM4 but not this MKII version that include a some enhancements like easy edit of velocity layers.
MatrixSynth, this is free. I've unsuccessfully tried to contact the author (Paul Le Secq).
Cameleon 5000: we only have a very buggy and incomplete version.

T-Racks in VST format

MachFive, Antares Filter and others are the "iLooked" ones so we have to forget them ...

Sonnox Oxford EQ seems to be for Authorized UAD cards only ...

The list can be continued ...