spellcheckers use them, any time you see a company with millions of users let you know your email address was already registered, domain name registrars use the

spellcheckers use them, any time you see a company with millions of users let you know your email address was already registered, domain name registrars use them -- basically anything with a large search space where you need to know if something is in (or not) the dataset. They are useful all over. I wrote a modified version of bloom filters so the filter files are about 1/3rd the size (but with a slight cpu tradeoff) for a domain name registrar. They are kind of a staple of computer science.