I use WP Digital E-Signature for integration with Sprout Invoicing. It works great. Before an estimate is accepted I have clients sign a document. They are then auto-redirected back to the estimate. Review collected by and hosted on G2.com.
I haven't found much of anything I dislike. The dropbox sync feature has never worked, but I feel that is likely user-error. Review collected by and hosted on G2.com.
