Phaser. Familiarize youself with effects and their uses.
If it's pure-effect you're after, then the EQ can do that as well, kinda. By setting it up so you have a drastic notch, say a mid-range boost/cut, then tapping the switch on/off repeatedly to a beat, you get a sort-of step filter effect, or "wah-on/wah-off" effect.
You'll prbably kill the switch doing that, but still...