1. Korg Monopoly
2. Cyclop This is the best bass synthesizer. It has the modern tone most people are looking for. There is a somewhat steep learning curve, with odd control labeling, but its worth using. There is not another plugin on the Mac thats quite like it. It is much better than NI Massive.
3. Heel Audio Mix Ref
This is a free Mac OS X plugin that is basically a speaker emulator. If you need to tame a sound or check an entire mix this gives you a virtual reference.
4. Amplitube 3
This classic plugin suite has so many good effects and amp simulations. Its a must have for any producer. The stomp boxes are useable, even with the rest of the chain bypassed.
5. Addictive Drums XLN
A very good interface for creating your own customized drumset. If you have no room to record drums this is your best option. The samples are very responsive. The level of control is very good.
6. Toontrack EZ Mix
If you have no idea what you are doing, and don't want to screw around with FX, this is an easy and fast solution. Its also a decent deal. Everything is labeled and it can make you more creative in record time. The level of FX control is not deep, but its not supposed to be. All the presets generally sound good. You don't need to learn a mixing tutorial with this plug.
6. Oxford Sonnox & Inflator
Great compression and mixing plugins. They make everything you run through them sound "better."
7. Izotop Ozone
I still use this for mastering when time is tight. The presets provided get you 80% of the way there, and with a little tweaking, make your mixes sound more commercial.
8. SoundToys & Filterfreak
A fun set of filters & effects that provide a wild character. Sounds Great.
9. Channel 3
This is mixer / channel emulator designed to replicate expensive consoles. Its kind of an audio exciter or EQ? It puts a nice touch on your sound. It is Free.
10. Elysia MPressor.
This is an excellent plugin for drum compression & pre-master effects. It can do the Daft Punk Homework fast attack and bring up the detail in your sounds.
11. Waves L3 Ultramaximizer. By far the best audio limiter for an extreme sound.
12. PSP Vintage Warmer Plugins Another must have for the end-chain in your mastering arsenal. This plugin inflates the stereo field and bumps bass.