A cheap-and-dirty spell-check that I did on the forums was to go into Google Analytics and export the titles of all of the pages with traffic in the last year or two. I put the titles into Word in plain text and used its spelling checker. I went ahead and fixed forum thread titles so the site looked better at first glance in the SERPs and in browsing the site. Because that site was user generated content, and I didn’t have the time and money to run a scan of all the content, I only fixed the titles and static areas and left the rest alone. Not something that can be done in every case, but it worked for me.

Xenu is just for the PC, so Mac users may want to look at Screaming Frog as a crawler. There’s a nice outline of what both Xenu and Screaming Frog do at http://www.seomoz.org/blog/crawler-faceoff-xenu-vs-screaming-frog.