VSTi plugins - Synthesizers
You will probably need some sort of MIDI Keyboard to run one of these properly, unless you want to use a virtual keyboard built-in to Pro Tools LE, Cakewalk Sonar, or one of the other professional audio-editing apps.
Spectrasonics Omnisphere, available from sweetwater.com $480
STEAM engine doesn't load down the computer - can run with no problems on 1st-gen MacBook. 8 independent arpeggiators, Groove Lock arpeggiator, Stack mode, 8-part multitimbral operation, up to 10 oscillators/patch, dual layer architecture, streaming sample playback, modulatable hard sync, granular synthesis, chaos envelopes, timbre shifting and crushing, FM, 6 LFOs/patch, 8 multi-breakpoint envelopes/patch, 17 filter algorithms, powerful modulation routing, up to 12 effects/patch, lots of signal-processing options, 1000 patches from Atmosphere can be downloaded into Omnisphere. you can dig from the simple interface deeper into the lowest levels of the architecture.
Sonic Reality Atmospheres, available from sweetwater.com $250
Collection of Virtual Instruments
Komplete 6, available from sweetwater.com $500
Everything you need for a studio - guitar stuff, Absynth (which can also re-synthesize existing sounds), Master, about 5 packages in all
Heavyocity Evolve, available from sweetwater.com $325
Phatbuzz (free version), available from trifex.de free or pay
boosts low frequency of your choice. 2 knobs: frequency and volume. does it without clipping.
Wavosaur, available from wavosaur.com free or donate
plugin that has plugins
VST plugins search from everywhere, available from kvraudio.com free and $