FileMaker GO has a great signature capture function. Others have made plugins for use with FileMaker Pro.
However, as far as i know, there is no good option for WebDirect.
It would be great for publishing quotes or some form of contract and allowing your clients to log-in and sign these in the browser.
Online singing can be (if done right and depending on the country) legally sound and is becoming more and more widespread. There are a lot of companies offering an online document signing platform, ie. Adobe Echosign, Rpost. However using a third-party platform isn't always an appealing idea. I believe creating your own version within FileMaker WebDirect would be easy to do with a variant of the GO signature capture module.
There are situations thinkable where the legality is too important to use this (but i fact GO faces the same issue). However I believe this would be an good enough option for most people.