If there isn't something in the appstore, you can easily build something yourself and even publish it in the appstore (other people would be thankful :) )
You can do stuff in java, but webservices would be easier. AFAIK, salesforce.com offers soap integration, just take a look @ the developer portal and here. I think Google offers also enough ways to easily integrate.
Regarding SFDC integration, we introduced Salesforce Connect in the AppStore for extending Salesforce.com - you can learn more about it and watch a demo video at http://www.mendix.com/salesforce