Chrome/Firefox: Have you ever started watching a long YouTube video only to get distracted and completely lose your place? Video Resumer is a simple extension that will remember where you stopped and pick up those videos where you left off.
Just install the plugin to Chrome or Firefox, and start watching videos. If you stop watching a video somewhere in the middle and leave the page, the extension will remember your place so you can pick up right where you left off when you return. It literally requires no other interaction. Hit the link to try it out.