The solution I chose in the end was Speller Pages, a very simple javascript-driven user interface with a number of different server-side options, each using the GNU Aspell application to do the actual spell checking.
Installation is extremely simple. Download the archive, place it on your web server (under /speller/ seems to be the quickest to get going), edit the JavaScript file to point it at either a Cold Fusion, PHP or Perl back-end, and add some extremely simple JavaScript calls to the page where you want the spell check to appear.
From the users point of view, all they have to do is click the obvious "Check Spelling" button next to the relevant textarea, and a logical, simple to use window appears highlighting the incorrect spellings and providing a list of relevant alternatives. Once they're finished selecting replacement words, the window automatically disappears and the user continues on their merry way - without having to worry about the embarrassment of sending their friends and family to a website full of typos or misspellings.
Total integration time, including making a new spell-check button on the page (with a bit of assistance from the icons by Mark James) , was about fifteen minutes. Not bad for a piece of functionality that makes life just that bit easier to use.