mesheconomy.com

Outlook plugin shows you most important messages first, Gmail style.

Rapportive shows you everything about your contacts right inside Gmail. Among other things, Rapportive also allows you to follow, reply and retweet from right inside Gmail.

Tout integrates with gmail, analyzes your outoing emails, and automatically creates templates to relieve the burden of recurring outgoing emails.

Techcrunch article

Adds rich information to your outlook contacts - including past info exchanged with that user. All updates on social media sites they participate on are also integrated.