The first tool is a Google Chrome extension...AdBlock. As you can infer, this app only works if you are using Chrome as your web browser. In order to explain why I love this app, let me share with you an anecdote you probably have also experienced...
I love incorporating short YouTube video clips to help explain the concepts I teach. However, I used to loathe being forced to show my students commercials before I could get to the actual video. During that 30 seconds, students can easily become distracted and I would often lose that teaching momentum that was leading up to the video. Alas, AdBlock removes this obstacle and lets you get directly to your video clip without silly distractions like commercials (especially inappropriate commercials).
FYI, a student showed me this great Google Chrome extension. If you haven't browsed through the Google Chrome Store, you are missing out on some really neat resources to make web browsing more efficient.
The second tool is something I discovered from EdSurge's weekly email called Clip Converter. This great little website allows you to download YouTube videos. If you work in an environment with spotty Internet access, you can download the video the day before you teach and have it ready for your lesson. If I was being observed by an administrator, I would definitely have my videos on hand in order to avoid any embarrassment.