I have just made available the Like and WP-TweetButton plugins to provide the Facebook “Like” and Twitter “Tweet” buttons for your blogs. Once you have activated one or both plugins, you can customize them from the admin dashboard, under Settings -> Facebook Like Button or Settings -> Tweet Button.
If you have used these plugins on another WordPress installation, please note that the defaults have been changed slightly so that the two buttons will show up nicely side by side without you needing to do lots of tweaking to your settings.