Classic digital signatures are used widely, but not explicitly by end users because the process is too complex. But the benefits of electronic signatures over paper ones are so clear that it was worth working around the problems.
This is what DocuSign has done with its eSignature software and infrastructure. Users provide an electronic image of their signature, typically with a tablet or smartphone. The system creates a signature card, analogous to those used in banks. When the user needs to submit a signature, he or she approves the submission from DocuSign to the other party. Electronic signatures such as these are legal under the Electronic Signatures in Global and National Commerce Act (ESIGN).
DocuSign provides iOS and Android apps, and a plug-in for Outlook. It also has a platform API with which it can integrate into larger systems. For example, it's integrated into Salesforce.com and all of Salesforce.com's salespeople use it. It also integrates into Microsoft Dymamics for CRM functions.