Edit: a new extension will tell you if you're running a shady extension.
I try to stay away from negative stories here, but this is information Sssimpli readers will want to stay on top of. On Friday, Ars Technica reported that makers of popular Chrome extensions were being approached by buyers who are not quite what they seem. These buyers, once they purchase an extension, are altering said extension to include adware and malware. Since Chrome extensions update in the background, it's easy to miss an extension's transition from useful utility to adware-filled nuisance.
Luckily, Reddit user smooshie has put together a list of the offending extensions, and other helpful Redditors are filling the thread with alternatives. If any of your currently installed extensions are on the list, you'll want to replace them immediately.
For your own peace of mind, know that Google intends to do something about the problem, albeit indirectly, by updating its Web Store policy to ensure developers keep their extensions' focuses narrow.