If you're looking for ways to expand your sonic palette without breaking the bank, there are some seriously heavy-hitting resources out there that won't cost you a penny. If you know of a decent VST that's not on the list, add it here!
The new kid on the block is Spitfire Audio's BBC Symphony Orchestra "Discover" edition. Fill in a questionnaire on their website, and two weeks later they will send you a download link to use with the Spitfire Audio app that you install on your mac or PC. (If you're the "I want it now" type, you can buy it for £49.) I've gone deep down the rabbit hole with this package, and trust me: you NEED it.
The Spitfire Audio app also lets you install a bewildering range of virtual instruments from their LABS range that includes treated string sections, granular synthesis guitars, toy pianos, whale song, and London foley. The fact that they're all free is ridiculous, given the quality of results you can get with them.
Want a Yamaha DX-7 but can't afford the prices they're going for on eBay? DEXED is so good an emulation that even Brian Eno uses it. And it will load "real" DX-7 ROM cartridge .SYX files, which are easily located online...
Cloudrum from Ample Sounds is a hang drum virtual instrument that I've been using for a couple of years now. It sounds lovely.
If you haven't dived into the world of SFZ samplers yet, believe me you're in for an adventure. You don't have to buy Garritan's products - good though they are - to use the underlying tech. First, you'll need to download Plogue's free Sforzando player:
Then you can start tracking down some of the amazing free sample libraries available. Once you've collected a few, load them into your DAW by dragging and dropping them onto the Sforzando plugin. You'll find everything from celli, drums, pianos and mellotron:
to "squid bagpipes":
to massive collections of both traditional and exotic instruments:
and full orchestras: