The good ol' Space Echo always works...but of course, they're starting to be a bit on the expensive side (unless you settle for the digital recreation, which is - imo - not bad at all). The Eventide Timefactor & Strymon's Timeline are both "pedals" that are crossing over into the "studio fx" territory, in both sound quality and versatility. Worth looking into.
Then again, pretty much any delay is nice with a 303 - you just gotta find something that suits your style. I've been using a combination of the Ibanez AD100 & some plugins (mainly Audiodamage's Dubstation) with my Bassline3.