Airwindows is one of my favorite plugin companies. It’s a one guy company with Chris making some great, simple to use cheap plugins in the AU format for Mac.
But he also offers a massive bundle of free plugins over on his site as well. If your wondering where to start, Chris recommends the following:
“Channel4 and Density2 are fine, but also look into Pressure3 to learn about compression, and DitherTo so you can learn how to use dither. SlewOnly and SubsOnly and Silhouette can also show you things about your mix, and Average gives you a kind of tone control your DAW won’t have. Try DigitalBlack2 as a gate and Mudslide on basses and you should have a lot of stuff to play with for free!”