The first question most people ask when we start talking about E-signatures is “are they legally binding?” I think so, but I'm not a lawyer.
With that said, in 1999, the EU passed the “EU Directive for Electronic Signatures” and on June 30, 2000, President Clinton signed the Electronic Signatures in Global and National Commerce Act (“ESIGN”), which made signed electronic contracts and documents as legally binding as a paper-based contract.
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There are quite a few providers, but here are some of the one's I'm familiar with. I've used most of them. I decided to go with EchoSign because it's an Adobe product and therefore should continue to be tightly integrated with PDFs.
Update: Ultimately I'd like to move to Docusign because it's more robust when you're using it on iOS. For now I'm using RightSignature because the pricing is better than EchoSign.
EchoSign (owned by Adobe)