The Digitech Digiverb pedal sounds very good for the price. Other than that, maybe an Alesis Nanoverb... a pricier and better way to go is the RealTube RV-1, the tube T-Rex Roommate or the EH Holy,Holier, or Holiest Grails. The Holiest is very complex and tweakable and can get some amazing, fascinating sounds.
However to my mind, nothing- not even a stand alone Fender spring verb unit- can take the place of a good spring reverb circuit built into your amp.
"There, in huge black letters, was 'The Grateful Dead'. It just... cancelled my mind out."-Garcia