Interesting web site for MD5 Online Cracking using Rainbow Tables can be found at http://passcracking.com/. Authors say:
This project is dedicated to crack md5 hashes online through web interface. At the moment we can crack md5 hashes in this character range: a-z;0-9  which means we can break almost all hashes (99.56%) which are created from lowercase plaintext with letters and/or digits up to length of 8 characters…
… This project is using RainbowCrack technology, which is based on Philippe Oechslin’s faster time-memory trade-off technique to crack the md5 hashes. We use 80 Rainbow tables each 610 Mb of size. So total size of the tables reach 47.6 Gb. Average time for checking one hash is ~40 minutes, but the speed increases as the count of hashes is getting higher. So generally the speed of cracking is ~150 hashes / 24 hours.
Also, visit http://www.antsight.com/zsl/rainbowcrack/ to find more interesting ideas.