Cirrus Insight

I think the best app to integrate Salesforce with Gmail. I would not use any other app. 

https://appexchange.salesforce.com/listingDetail?listingId=a0N30000004fnkUEAQ

Have a look at their demos. 

http://www.cirrusinsight.com/features/

Appirio

Cloud Factor

http://www.cloudfactorapp.com/

Appirio Cloud Sync

https://appexchange.salesforce.com/listingDetail?listingId=a0N30000004cg4GEAQ

Groove

https://appexchange.salesforce.com/listingDetail?listingId=a0N30000009wgl9EAA

I haven't tried this - but it is $10/month and says it works with any field and any object. And it has very cool Email template integration. I must try this app. 

Automated Emails

Apps that will move all emails into Salesforce. 

Note: I have yet to come across a company that this sort of app works for. And they normally have to send all your corporate emails to their servers, which is not a good solution. 

Match My Email

https://appexchange.salesforce.com/listingDetail?listingId=a0N30000003JuTlEAK

I've had a demo of this app and it is great. If you do want this sort of app, take a look at MME, it works really well. 

IHance

https://appexchange.salesforce.com/listingDetail?listingId=a0N30000003GxgGEAS

Floating Apps

http://www.floatingapps.com/salesforce-gmail-integration/