Data Collaborative integrated an e-signature into my app. It was a great experience and their product is top notch. I would recommend their products over others simply because they have a one time purchase model and their product works flawlessly.
DC Insight
Product Description: 
This Application Documenter Tool allows you to document the purpose and interrelationships of fields both when you're building a Quick Base application and as it gets modified over time.

Pricing for On-line version of the application:
$59.95 per documentation session
$39.95 per documentation update (requires a previous document)
$34.95 per token, prepaid when purchased in blocks 5 or more.
Support via email only. If support is product bug related, no charge, if it is consulting/usage related, normal consulting rates apply.
Installation:
Our standard rate of $165 per hour applies if circumstances prevent a normal installation, or if assistance is needed in setting up Quick Base and client meetings are required.
Read More