Personally, I'm a huge fan of Nautilus. It has tabbed browsing (File->New Tab or CTRL+T), allows some layout customization (particularly if you're willing to edit more advanced settings in dconf), and has a large selection of plugins available. In Ubuntu, you can view the available Nautilus plugins with Aptitude (aptitude search '^nautilus-')
or by searching for "nautilus" in Synaptic Package Manager.
If I had to choose another file manager, though, it would definitely be PCManFM