Suspicious activity in Gmail

Question

gmail

Recently, I began to notice suspicious access to my mail via the IMAP protocol from American IPs. Here's the actual screenshot my.jetscreenshot.com/2642/20120425-htdg-79kb.jpg
Access happens at the moment the browser is launched, but not every time. What could it be? How dangerous is this, and how to figure out the culprit?

Answer 1

Change your password, enable two-factor authentication, scan your computer for viruses

Answer 2

I had this with Greplin. Haven't registered with them? You can disable auto-sync on their website.