Besides Tabs Studio there are other Visual Studio tabs add-ins, though none of it is in active development now:
- WndTabs by Oz Solomon. Supports Visual C++ 6. I was a happy WndTabs user myself until Visual Studio .NET came out. Source code is available.
- VSTabs by Jonathan Payne. Supports Visual Studio 2003. Very small feature set. Source code is available.
- EasyTabs by Sven Ritter. EasyTabs for Visual Studio 2005/2008 original announcement in German, English translation. Supports Visual Studio 2005/2008. Developed on the basis of WndTabs. Source code is not available, but download is free.
Tabs Studio Blog (organizing Visual Studio document tabs) 
Sadly, the vstabs website is gone 😦
Comment by Legolas — March 23, 2010 @ 7:49 pm
Sadly indeed 😦 I’ve removed the link.
Comment by Sergey Vlasov — March 23, 2010 @ 9:56 pm