Definr.com is a fast, free dictionary based on Princeton's open WordNet 2.0.
The website framework is Ruby on Rails. Looking up words in a dictionary is easy, so we do that with MySQL and cache the top 10,000 definitions in memory.
Doing word completion (a.k.a. auto-complete, auto-suggest, globbing) is not easy, at least not when you have to search through 200,000 words for every keystroke. So our word completion is done in a C module for Ruby, and the word completion server is separate from the rest of the site.
With the DefinrBot algorithm, we can do 190,000 word completions per second without caching, and that translates to about 10,000 completions per second once the Ruby layer is factored in.