I use Twitter. A lot. Twitterfon is likely the most-used app on my iPhone. I have a Twitter widget in my MacBook's dashboard. And Twitter itself is one of my browser's home tabs. I make sure I've got a Twitter client of some sort on all of my devices.
Now, thankfully, I can tweet from my Commodore 64 using the adorable app called Breadbox 64 (yes, guys can say "adorable"). Of course, your Commodore needs an Internet connection, but as developer Johan Van den Brande points out, it's all very doable. In a post on his site, the Belgian engineer gives a step-by-step and even offers the app for free download.
Not only does Breadbox 64 allow you to tweet, it also lets you access your friend feed, as well as individual friends' timelines. And, of course, Van den Brande also has details on the hardware, like the MMC plug-in ethernet adapter, as well as various screenshots. It looks pretty awesome.
Sadly, I don't really have a Commodore at home. But if someone can get Twitter running on a Texas Instruments TI-99/4A, I'd be a very happy guy.