I responded to your email as well on this. You are more than welcome to use our API's for the purpose of integrating with SugarCRM. Everything you need to use our API's is at developer.constantcontact.com. You can also use our Marketplace to find developers.
Partner API Support Engineer
I'd also like to point out that there are some out of the box integrations already available (some at an additional cost) through our MarketPlace as well: http://marketplace.constantcontact.com/AppsSearch?query=sugarcrm