The first is LFO Tool by XFER. LFO Tool basically allows you to shape any sound you want the way you want it to and runs two filters making it fucking incredible. its only 25$ (i think, it may have gone up to 50$ now) and is so versatile it's not even funny. You can get bass wobble sounds as well as shape eighth note plugs the way you want them to.
The second is Dada Life Sausage Fattner. This shit is fucking awesome for bass sounds that need more firepower behind them. only 20something $$
The third is bitspeek by Sonic Charge. wanna make any vocal file sound vocoded as fuck and robotically awesome? this plugin will do that for you. The most notorious use of this plugin is in Kill the Noise's "Kill the Noise Pt. I" 29$ i believe
Also, WAVES bundle is really sick for anything mix related.
There is also a really great metering plugin called FreeG by Sonalksis which allows you to see the RMS levels as well as your db on any track. they have one for mono and stereo and they are both free on their website.
Any plugins you guys have run into that you think everybody should know about?