Digital Contract Services and Mobile App Signing

HelloSign
This is the only prominent digital contract service that appears to offer a free version of their service.  It's limited to three contracts per month and does not have templates, but if you're booking more than three contracts per month, $13/month (when paid annually) should not cut into profits too much.  For the wedding industry, where gigs are at least $1,000 each, this should be a reasonable cost to have contracts signed faster and avoid the time and labor of mailing and signing paper contracts.


Make Your Own Web Form / Fillable PDF / or Google Form Submission
To save on headaches of signing up for a new service or paying extra to use a premium service, create a custom contact form on your website that has fields for the bride to fill out and submit to you.
Contact Forms (with the right Wordpress plugin) can be easily duplicated and re-purposed for the next client.

After the client submits the contract form, copy the content into a Google Drive document, then make a PDF and send a copy to them so they have it.

ShootProof
They offer a 30 day free trial of their service, and there is also a free plan to host 100 photos.  I'll find out after the trial expires if the Contracts service is still available, but it appears that the all the features are available, as long as the user doesn't exceed the maximum number of photos for the respective plans.

Update: contracts are not included with the free plan and when I went back to finish my contract, it was hidden (probably until I paid for a subscription).

DocuSign

Their service's cheapest option is $10/month when paid annually. Five documents per month and a Mobile app should offer what small businesses need.  The 'Personal' plan does not offer branding, so larger or higher grossing businesses may want the 'Standard' plan at $25/month.

HoneyBook
A young professional wedding photographer I know uses HoneyBook for her wedding clients. It's one of the more expensive options, but their service provides a lot more than mere contracts.  They also provide online invoice and payments and project management.  If their project management and expense tracking is good, then it would be worth the $40/month to have an all-in-one solution that could replace or consolidate two or three other online services.

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Apps for Signing Digital Contracts

Shake (featured in 2013 on Gizmodo)