It lets you highlight a portion of a webpage, send someone a link, and then they can see exactly what you highlighted. Great for virtual reference work, but also just good in general.
One of our more tech-savvy patrons emailed me asking if there was an easy way to search the Library's catalog right from book's page on Amazon. There is, using Firefox and Greasemonkey, and it is outlined on my Library's Tech Tools page.
But instead of just sending him the link to the Tech Tools page, I ran it through the Awesome Highlighter, so I could send him a highlighted page, with focus on exactly the portion of the page I wanted him to see. Not that he wouldn't have found it on his own, but it just makes it a little bit easier - especially the "jump to highlights" link at the top.
On the Ning page, there's some discussion about the highlighter working or not working depending on whether the user is signed in. I've only used it a couple times, but I haven't had any trouble. The great thing is that someone from the company is participating in the discussion, so hopefully whatever bugs do exist will be corrected as a result - much like Jessamyn's comments on SWIFT.
If we never speak up, then we'll never get tools that do exactly what we need (I'll refrain from inserting my ILS soapbox here).