Just curious which VSTs are on your 'couldn't live without them' list. Currently mine has (more or less) stabilised on:-
Free:
ePiano
Mr. Ray for Rhodes/ Mr. Tramp for Wurlitzer. Mr. Ray 2.2 is particularly good, and although possibly there are better emulations these seem to come close enough - especially for the price.
Hammond Organ
Nubi and Spinner. If you can find it, the later 'plus' version has a particularly wide range of controllable parameters
Organized Trio is also well worth the zero cost
Synth
Crystal - seems to be capable of a wide range of sounds - there are many other free alternatives in this area too, but this is rock-solid and fairly simple to program.
Sonar stuff
Although I no longer use Sonar, Perfect Space from Sonar 8 is still very definitely a must-have. Dimension Pro has also survived - it does have some wonderful pads, though many of the other samples are a bit weak in comparison to Kontakt. Rapture unfortunately I had to give up on. It is too unstable and takes out whatever host, including Sonar, I use it with. (at least the LE version bundled with Sonar 8).
Effects
A bunch of excellent free plugins from Voxengo and others. Seems to me if you need compressors, EQ and reverb you don't need to be paying for any of these unless you have pretty specialised requirements.
Non-free
Guitar
I will never regret purchasing Musiclabs Real Guitar, Real Strat and Real LPC. These are probably the most played instruments I have. Plugged into Guitar Rig 3 LE (as bundled with Sonar) they are incredibly playable and entirely convincing. And totally solid and glitch-free.
Drums
EZDrummer. And Geist. Together they make the world's most powerful drum machine. Geist's samples tend to the electronica/hiphop end of the spectrum but the pattern and sequencing functionality are just awesome, and it is quite capable of triggering any other drum samples or plugins.
Piano
I have two PMI sample sets; the Bosendorfer and the Steinway. Between them I couldn't ask for more. They are played back via Kontakt.
Sampler
Kontakt. NI finally offered an unbeatable crossgrade deal. It is a superb sampler and the bundled instruments are of very high quality.
Orchestra
I have Garritan Personal Orchestra and it does complement Kontakt quite well.
Wishlist
I would like a decent choir - especially one that could sing whatever I wanted - but that's not essential. It's disappointing that Vocaloid never got particularly convincing, it'd be amazing to have a computerised vocalist.
Also a decent pedal steel guitar would be nice.
So what's on your 'must have' list?. I'm trying now to stop just adding things - you end up wasting so much time. And I have several real physical synths, including a Yamaha FS1R, which really is unique, and an old Kawai K5m I picked up for next to nothing in a charity shop, but which again is a unique instrument. So I'm trying to focus on getting the best out of what I already have. Unfortunately, always open to temptation......