Yeah, we need to know more about what you want to do for specifics... but generally speaking a couple outstanding low cost units... check out the Alesis Midiverb 4 and the Lexicon ReFlex. These have many useful features and sound great regardless of how you plan to use them.
The ReFlex is about ten years old, so you'll want to look on eBay, Craigslist, etc. Sound wise it is IMO head and shoulders above the newer "budget" range from Lexicon.