Basically TweetMeme gives bloggers tools to make it easy for their viewers to post to their Twitter accounts directly from their blog.
reBuzzThis does the same thing only for Google Buzz, it's less featured than TweetMeme, and only tracks info for reBuzzes through their site and buttons, however it's a start and I expect it shall become big just like TweetMeme.
For bloggers here are the buttons you can get for your blog or website.
My personal Favorite /\
again this comes in blue yellow green and red