This is what I found:
Description:
A reverse DNS lookup shows ContextWeb using ContextAd Bot. You should only see this bot if you display advertisement on your website. ContextWeb might be on the Google Certified Ad Networks list and if you display AdSense ads it could explain the visit. If you have blocked the Google Certified Ad Networks in your AdSense account and don't have any advertising agreement with ContextWeb you shouldn't see this bot on your site. The 2 IP addresses below have been the only ones we found so far.
So it looks like this is an AdBot/Web Crawler of some sort.
