Its security is based on the fact that it is a destructive algorithm: if you MD5-encrypt a string, you’ll get an another 128 bit (16 characters) expression that is the unique checksum of the original string, and it’s impossible to go back. The result can be compared to the MD5 of another string (e.g.: entered by a user); if the two checksum are exactly alikes, the input received is correct.

This is a good way to securely store users’ sensible information (like passwords), making them not understandable by anyone but still usable to verify user authentication.

MD5 reverse engineering is not possible (you should calculate MD5 of any possible string or phrase to find the correspondent original one), but several websites born trying to do the impossibile, storing MD5 hashes and original strings from anywhere.

The utopic idea: if we’ve a database of MD5 checksums of “everything”, we have a “dictionary” to recover any expression from its MD5.

… maybe just another useless Internet application. Do you want to contribute adding the checksum of your name to the World’s MD5 database?
Mine is 9667aacee4c11ab5cb1aee39cb183599

Biggest MD5 crack databases
http://nz.md5.crysm.net/ (28,000,000+)
http://md5.rednoize.com/ (47,000,000+)
http://gdataonline.com/seekhash.php (168,000,000+)
http://www.tmto.org/?category=main&page=search_md5 (306,000,000,000+)

More about MD5
http://en.wikipedia.org/wiki/MD5