Aspeller is the name I've given to my latest script. It's basically an interface between aspell (the OSS *nix spelling program) and Irssi, highlighting misspelled words as you type. It also offers a command that responds with a number of suggestions for a misspelled word. It's still in development, but it seems to run fairly well on my 300Mhz box ^H^H^H beast.
All the associated documentation can be found at the beginning of the script, which can be downloaded here. It is of course Open Source, under the GPL v2. Feel free to download it and alter it to your hearts content. It has only been run under linux, although it may run under other operating systems.
Aspeller is highly configurable. All colours involved can be specified, as can the rate at which it scans the input line. It uses its own seperate personal dictionary, which keeps words acceptable for irc independant of words acceptable for important documents like resumes.