(1) single column (this is pretty ugly)
(2a) two equal-sized columns
(2b), (2c) two columns: a narrow one and a wide one (there are two flavors for this layout)
(3a) three equal-sized columns (this is default one)
(3b) two narrow columns and a wide column in the middle
(4) four equal-sized columns
I think the most interesting layouts are the ones that combine different types of columns, so you can have more space for the gadgets. The last layout is great for wide-screen computer monitors.
To edit the layout of a tab, click on the small arrow next to its name and choose "Edit this tab". You have the option to rename it and choose a new template. For now, there's no option to change the layout for all the tabs at once.
{ via Google Blogoscoped }
