I wonder why nobody posted it yet as quite a few people here seem to use it, but I'll post it now so that even more people know about it. =)
Name of VST: String Theory v1.5
This VST does: Emulate any kind of strings and strings-alike instruments
Quality: Seems very good to me, and also very inspiring.
License of this VST (and costs): Freeware
Link to VST or site: http://www.ugoaudio.com/Name of VST: Rez v2.0
This VST does: Synth stuff, I found it very good for synth basses
Quality: Well, yet another softsynth. I found the presets of this one also very inspiring, and you seem to be able to do a lot of great stuff with it. However, it seems to cause OpenMPT to stutter every now and then.
License of this VST (and costs): Freeware
Link to VST or site: http://www.ugoaudio.com/