Twitter users can now collectively celebrate Earth Day with the Tweet-a-thon. Called Earthtweet, the online event gathers planet-lovers through the hash tag #earthtweet.

What is a hash tag, you ask? It is a short code (preceded by the pound sign) that basically acts as a syndication device (like RSS) on Twitter. So each post that contains the characters #earthtweet will be pulled together into one feed and posted on the Earthtweet.com website.

Users are encouraged to post up the things they are doing to celebrate Earth Day -- like keeping the lights off or taking a group shower (though that never seems to end up saving water) or biking to work. Especially active tweeters will receive prizes offered up by the tweet-a-thon community of businesses.

The organizers explain their mission: 

With a new administration and a renewed focus on the environment, Earth Day '09 is bound to be momentous. Use the #earthtweet tag to share what you're doing to help celebrate Earth Day. Whether you're hosting an event, or coming up with a unique way to conserve, your earthtweets will inspire others to get involved.

Tweet away for Earth Day!