Twitter Gadget is a great app for accessing Twitter, even if Twitter is blocked (like it is at my district).
Twitter Gadget is an app that you can use in iGoogle, GMail or as a stand-alone app. It is very easy to use and works well. I actually like using it in iGoogle over the Twitter home page.
It's free and the site has easy instructions and references on how to use it.
TwitterGadget in iGoogle:
TwitterGadget Stand-alone in browser:
It's a great resource and I use it even when Twitter isn't blocked.
(If you use a Palm Pre+/Pixi+ I recommend Twee as a great Twitter app for webOS)
Any other great Twitter clients that get around Twitter being blocked?
UPDATE 12-10-2010
TweetDeck and Tweetdeck's web app also work if Twitter is blocked and are both very powerful!