There are a number of open-source and shareware applications that allow you to download video from YouTube for offline viewing. YouTube Downloader from Bronenos blows them out of the water.
It adds buttons directly above a video on YouTube's site (but not videos embedded on other sites) that let you download in either FLV or MP4 formats. This is a great option for later video viewing or for building YouTube content into presentations or multimedia projects (while obeying copyright and/or fair use rules, of course).
YouTube Downloader isn't available in the Safari Extensions Gallery, but you can download it from the developer's site.
Reference, search, translation and more
This versatile reference tool from Apture Inc. is especially great for students. Once it's installed, anytime you highlight a word or phrase on a Web page, it displays a pop-up window in which you can search for additional information about the selected phrase from Wikipedia, Google, Bing, YouTube or Google image search.
When you press a key combination -- you choose the combo in its preferences -- Canisbos Computing's PopSearch extension displays a "pop-up" search box overlay that lets you search any of 14 different sites. That list includes obvious ones like Google, Bing and Wikipedia as well as less obvious choices like Amazon, your Instapaper account, Twitter and Facebook.
Félix Cloutier's Search Preview adds inline thumbnails of linked pages to your search results from Google, Bing or Yahoo. It's invaluable for getting some context about the pages included in your search results. Search Preview isn't available in the Safari Extensions Gallery, but you can download it from Softpedia.
UseKit is a very handy add-on that rolls up the features of several popular extensions and services into one. It adds a toolbar to the bottom of your browser Window with buttons for several functions, from language translation to Web editing. Here's a list of the services included:
- Translate: Translate any text you select on a Web page to or from English and six other languages.
- CrunchBase: Look up company names in TechCrunch's user-editable online directory of tech industry companies, people and investors.
- Map: Look up any address on Google Maps.
- Share: Share a link to the current page using e-mail, Facebook, Twitter, Google Buzz, Digg, StumbleUpon and/or other services.
- Printliminator: Remove unwanted images, paragraphs or page elements for printing.
- FireBugLite: Explore the code underlying any Web page.
- Readability: Apply a more readable template to any page, removing ads and other clutter. (Safari's built-in Reader function does much the same thing, and more elegantly, but on single article pages only.)
If you want a more complete translation tool than the one provided with UseKit (or if you're not interested in UseKit's other functions), give Side Tree Software's Translate a try. It taps Google Translator to translate the text of any Web page into one of nearly 60 languages (the default being English).
Perhaps the best feature of Translate, which adds a toolbar button to Safari, is that it doesn't just translate the text, it re-renders the entire page with the translated text and (optionally) displays it in a new tab.