http://www.hashcash.org/
Hashcash is an anti-spam technology which requires the sender to compute a partial hash collision (which takes a predictable amount of time) in order to "pay" for their email. On the flipside, it's very easy to verify the hash.
http://www.codinghorror.com/blog/archives/000953.html
An article on CodingHorror.com about how storing passwords as simple hashes can prove to be very insecure. Some recommendations are made including using a long salt value.
