We're a Salesforce Foundation partner that uses FormAssembly for quite a few of our implementations. One feature we'd really love to see is the ability to do a single sign-on for Form Assembly and Salesforce.com. Basically, the end user would sign up for FA and associate it with their Salesforce account. Then, whenever the user signed into SFDC, they'd also be signed into FA. This would allow for seamless user experience from FA to SFDC and back again.
This is currently possible when a user installs our AppExchange app, available at http://sites.force.com/appexchange/apex/listingDetail?listingId=a0N300000016ac6EAA.