Another great small budget movie is Primer (2004).
It has one of the biggest Budget to Box office multiples I know of: ~120x ($7k -> $841k)
I think Mad Max holds the record. (the original)
Mad Max. $200k budget, almost $100m worldwide, for a 495x ROI
Deep Throat had at least a 600x multiple (a budget of $47,000 and a box office somewhere between 30 million and 50 million). Blair witch had over a 1000x multiple, with the same 200k budget as mad Max and a box office around 200 million. Paranormal Activity also seems to be close to the 1000x mark.
EmpLemon has a video about this. He argued Blair Witch had the highest multiple (~1200x)