In the implementation in Visual Studio 2010's "Tab Well" (a feature of the "Power Tools" extension), pinned tabs can be multi-row, non pinned tabs are in one scrolling row.
In Firefox's "Tab Mix Pro" tabs can be multi-rowed.
In "Tab Mix Pro" I can always easily and quickly tell which is the active tab.
In "Tab Well" it can take me a while to figure this out.
What are the things that should and shouldn't be done in multi-row tabs?
If you intend on answering this, please don't automatically assume that multi-row tabs are bad - keep an open mind.
Here supported sites have there own icon and selected tab is emphasized, new tabs have red text. Tabs do not have a border that continues from header.
Firefox's TMP - with tab selected on top row
Firefox's TMP - with tab selected on bottom row
VS's Tab Well
Here each project has its own color and selected tab is slightly emphasized. Selected tab has a border that connects to the header (assuming header is bottom row - I find this confusing if selected tab isn't on bottom row).
VS's Tab Well - with a red tab selected on bottom row
VS's Tab Well - with a green tab selected on bottom row
VS's Tab Well - with a pinkish tab selected on middle row
VS's Tab Well - with a green tab selected on top row
An old settings dialog
This is from Word 6. I think I remember seeing this in Win95 settings too.
Here the tabs rows swapped if you click on the top row. Which was confusing, because if you click on were the previous tab was to go back, it was no longer there. (Thanks to Andre for the reference.)
This image's animation shows the different states.