Chrome/Firefox: If you're on a slow internet connection, YouTube is unbearable to use. It never seems to know when to start playing and when to stop buffering. SmartVideo for YouTube gives you control over all that.
SmartVideo gives you control over YouTube buffering in all sorts of ways. You can set it up so the buffer has to fully load before the video will play, or leave it on the "smart buffer" that decides when to start playing the video based on your download speed. There are a few other little options built into SmartVideo, but its main appeal is the minute control you get over buffering. If you're stuck on a slow connection, it makes YouTube a lot more bearable.
SmartVideo for YouTube [Chrome Web Store]
SmartVideo for YouTube [Firefox Add-Ons]