One of my favorites is Flashblock. Under Unix machines, it seems that the Flash interpreter is not well optimized and, depending on the actual Flash content, can really slow your box down. This can be really irritating when the Flash is some ad or incidental thing unimportant to the page. Flashblock puts a placeholder in place of the Flash. If you want to actually run it, just click on it. The only problem is that sometimes you have to be a fast clicker in order to not fail a check-for-Flash test.