If you were online last night, you likely experienced repeated messages from Internet Explorer, announcing it had to abort the attempt to access select sites. The cause? Sitemeter. Sites with the Sitemeter icon were affected, which given my experience last night and the seemingly odd randomness of the blocked sites, now makes sense. Read here for Sitemeter's statement. The problem has been fixed.
If this happens again, I now know there's an easy fix: Block Sitemeter from your list of sites (ah, if only we had known this was the cause). Or do what I did: install Firefox.