Little Foot (on sale for $19, marked down from $39) makes it simple to add girth to any kick drum in a way that can't be done with EQ and compression, simply by dialing in the frequency and sustain.
Mongoose (on sale for $15, marked down from $25) lets you narrow your bass frequencies to give a low end that sounds more natural on monitors and headphones. It also allows you to widen your stereo image without introducing that head turning low frequency width.
Bark of Dog (free) is a resonant high pass filter that lets you cut your low frequencies without thinning out your track.
The sale ends on March 20, 2014.