It depends on how far your are into your music-production journey, but for a few power tools:
- Cardinal (free)
- PlugData (free)
- Chow BYOD (free)
- Vital (free)
- Surge (free)
- UVI Falcon
- Voltage Modular (has a free tier, and an active Humble Bundle upgrade for $1 as I type this)
- Redux (of Renoise 'fame')
- BYOME (UA / Plugin Alliance)
- TRIAD (UA / Plugin Alliance)
- Pretty much any of Glitchmachine's plugins (they usually go on sale for about $10 a pop)