This collection of popular Greasemonkey scripts makes tweaking Gmail's look and behavior as simple as installing a Firefox add-on. The 15 features in Better Gmail 2 allow for a deeper level of customization than Google can provide, even though the introduction of Google Labs in Gmail demotes Better Gmail from "must-have" to "still pretty cool."
The features in Better Gmail 2, labeled as such because of Gmail's upgrade in late 2007, could be essentials, depending on what you get out of them. Perhaps the coolest feature is the one that summarizes unread messages. There's also mouse-over row highlights, representative attachment icons to replace the paperclip, and an icon tweak so that those attachment icons are system-accurate. You can list Gmail's labels in a folder-style hierarchy, force your in-box count to appear on the Gmail tab title, or force your unread message count to appear in the Gmail favicon--in two different styles.
The need for Better Gmail 2 has been slightly mitigated by Google introducing its own customization solutions. If you're looking for one of the specific features that it provides that Google doesn't, it can still be a useful tool in your Firefox add-on kit.