Jonathan Ellis's Programming Blog - Spyced: All you ever wanted to know about writing bloom filters
http://spyced.blogspot.com/2009/01/all-you-ever-wanted-to-know-about.html
pHash.org: Home of pHash, the open source perceptual hash library
http://www.phash.org/

a fingerprint of an audio, video or image file that is mathematically based on the audio or visual content contained within. Unlike cryptographic hash functions which rely on the avalanche effect of small changes in input leading to drastic changes in the output, perceptual hashes are "close" to one another if the inputs are visually or auditorily similar

A method for producing hashes so that similar objects produce similar hashes. Applied mostly to audio and video. Doesn't appear to be a general theory. Rather, special cases for files such as pictures and audio are derived. This is an open source library that runs under linux only at the moment.

I know very little about cryptography, but I do find it fascinating. This article seems to have solid, real-world advice, yet it is written in a way that even I can understand it. People who can write like this impress me.

why hash is not security

I mentioned in a previous post that I once interviewed for a job at a well-known search company. One of the five people who interviewed me asked a question that resulted in an hour-long discussion: "Explain how you would develop a frequency-sorted list of the ten thousand most-used words in the English language." I'm not sure why anyone would ask that kind of question in the course of an interview for a technical writing job (it's more of a software-design kind of question), but it led to a lively discussion, and I still think it's one of the best technical-interview questions I've ever heard. Ask yourself: How would you answer that question?

I mentioned in a previous post that I once interviewed for a job at a well-known search company. One of the five people who interviewed me asked a question that resulted in an hour-long discussion: "Explain how you would develop a frequency-sorted list of the ten thousand most-used words in the English language."

The author talks about a question he got at a job interview, and goes on to provide a reasonable recap/discussion about hash tables. This is generally the kind of answer I look for when I ask similar questions. 9/10 candidates I talk with can't actually discuss a hash function, and don't know how to create one.