Gmail has added a Labs feature for nested labels. Now, you no longer need a Greasemonkey script to get the effect, it's built right in. Google announced it earlier today on the Gmail Blog. Getting the effect is easy, just name your labels like a folder structure (projects, project/gizmo, projects/widget, etc...) and they'll appear as nested in the label sidebar, but not the expanded additional labels box.
They also added a cool "Message Sneak Peek" feature which lets you preview and take action on messages without leaving the inbox.
Both look to be really useful, and if you haven't spent some time looking at all the features in Gmail Labs, now would be a really good time to start.
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