1. YouTube Feather (beta)

The "Feather" project is intended to serve YouTube video watch pages with the lowest latency possible. So basically when you goto YouTube and select a video, it will load the page with minimum content. There will be only the search bar and the video. No ads, no suggested videos, no comments.

2. SmartVideo for YouTube

Have you noticed that when you pause a video on YouTube it also pauses loading the video. This can be pretty annoying if you have slow  internet and you are trying to let the video buffer before start playing the video so it doesnt stop half way. SmartVideo for YouTube is an add-on you can install to your chrome or firefox so it makes sure the video stays buffering even while the video is paused. It has some other handy features like setting up a preferred video quality, player size etc too.