Does anyone know of any Appian applications that support digital signatures / e-signatures?

There is the Sign PDF Document plugin:
forum.appian.com/.../summary

I'm doing some research on this subject at the moment and from what I've gathered it should be possible as long as the signature platform that you use provides a web service (SOAP or REST) that Appian can consume. However, you must consider the security concern/risk that comes with storing the credentials in process variables from the time you capture the credentials in the SAIL form to when you receive a response from the Web Service.

DocuSign Web Service Calls can help. Try getting REST WebServices for DocuSign. I think they have them. For credentials, I believe, SAIL now has encrypted text and you can always use the crytography plugins to encrypt and decrypt your passwords while passing into the process variables. Make sure your algorithms are very strong.

What is your exact use case. Major eSign vendors like Docusign/CIC have restful services available and integration with those are pretty straightforward.