As much as I love using Gmail, the Greasemonkey script-based Better Gmail plug-in made my favorite e-mail client, well, better. Integration with Google Reader, colored labels, built-in TinyURLing, and other features really made Gmail that much more useful. Since Google's recreated and upgraded most of Gmail's back-end, those scripts and most other Gmail plug-ins have broken.
To confuse matters, Google is slow in rolling out the new features, so not everybody is using the new interface. If you do have the new Gmail, you can access the old interface Better Gmail 2 v0.1 recreates some of the features in its predecessor, and promises to eventually feature most if not all of the old expanded functionality. For now, though, it's slim pickings.