Mac/Linux: If you can't get Flash working right, or just like a Flash-free viewing experience, Minitube is a full-fledged desktop client that searches and plays YouTube videos, in HD when available, in a very minimal, sleek interface.
Minitube isn't a complicated app. There's a search bar when you first start it up, a list of results on the left, and a video window on the right. You can toggle HD viewing on and off, switch to full-screen mode for longer clips, and scan through the video — or open the YouTube page, if you need more context. That's about it, but if you're looking for a way to check out YouTube clips outside the browser, it's a really elegant solution.
Minitube is a free download for Mac and Linux systems.