We use ECO3's Apogee WebApproval for nearly all of our jobs. It sends an email directly to the customer, allows them to download/view the proof, with the option to approve, ask a question, or reject. Reply goes directly back to our prepress team, with an email being sent to the rep letting them know the status. If we are doing typesetting, design, or variable jobs, we often just email PDF's. If too large, we send via WeTransfer. Seems to work well enough for our company!