Mozilla releases Thunderbird 3 Beta 4
Gmail integration now includes syncing for your sent mail and trash folders, and uses All Mail as the archive folder. Smart Folders allow you to combine special folders (like your inboxes, and the aforementioned sent and trash folders) from multiple accounts. Advanced Filtering lets you search by a specific category, including by sender, tags, and attachments. Sure, there might still be something you can do in Gmail on the web that you can't do in Thunderbird, but that list seems to be shrinking with every new beta.