Jeff Atwood writes on his blog Coding Horror  about a password cracking technique that uses rainbow tables.  The premise is simple, taking advantage of the time-memory tradeoff of storing massive amounts of pre-computed hashes in memory. 

The multi-platform password cracker Ophcrack is incredibly fast. How fast? It can crack the password “Fgpyyih804423” in 160 seconds. Most people would consider that password fairly secure. The Microsoft password strength checker rates it “strong”. The Geekwisdom password strength meter rates it “mediocre”.

Coding Horror: Rainbow Hash Cracking