http://pajhome.org.uk/crypt/md5/
A javascript implementation of md5. Very handy.
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.thenewtownhash.org/
Hash House Harriers club in Edinburgh