TheMasker

Though the developers say there may be a few bugs, this works perfectly for me in Reaper. Probably the best free alternative to Trackspacer you can find. This video here helpfully explains the features in depth, but here's how I use it on kick and bass:

• The bass is clashing with the kick, so I put TheMasker on the bass bus and send the kick to the sidechain input.
• Set 'MASKED FREQS' to bury.
• Adjust 'SIDECHAIN LEVEL' so the effect is actually triggered.

The result is that whenever the kick is playing, TheMasker cuts any clashing frequencies on the bass.