What is reverse delay?
A reverse delay takes your signal and kind of flips it around. What comes out of the pedal is a sound that makes the impression that what you play is being played backwards. The tones kind of fades in, just like they would do if you were to record them and then reverse the track, playing it backwards.
Any good examples?
It all started when John Lennon came home one night in 1966 and wanted to listen to the Beatles song "Rain". Lennon accidentally put the tapes backwards, so what came out of his speakers was the song backwards. Lennon loved the result and talked producer George Martin in to doing this as the outro of the song. After that Beatles experimented wildly with backward effects, something that can be heard on the guitar on "Tomorrow Never Knows" and "I'm Only Sleping", and on the drums during the verses of "Strawberry Fields Forever" (the whole kit is reversed here, but later on reversed cymbals has become a great production trick on many songs), among else. Jimi Hendrix liked what he heard and used the same effect on his guitar solo on the song "Castles Made Of Sand". Since then many guitarist have used the reverse effect, one example is John Frusciante on the Red Hot Chili Peppers song "Give It Away".
Which pedal should I get?
These days many delay pedals also offers a reverse mode, like the Boss DD-7, the Eventide TimeFactor, the TC Electronic Nova Delay or the Korg SDD3000. Also, some loop pedals has this mode. But if you're looking for a more unique reverse delay effect pedal Danelectro offers both the Back Talk and the Wasabi Forward-Reverse Delay Pedal, and seems to be pretty alone in the reverse delay business. They might not be especially versatilite pedals, but they're really cool units for sure.