The AJAX controls from
www.asp.net are decent and there is a tab control.
Obout has a few free controls, but not a tab control (
www.obout.com). For
tools, controls, etc.
www.sharptoolbox.com is a great resource. aspin.net is
also a search engine, although I do not use it as much any more.
I have not tried, as Mark has, for free controls search, but I am sure there
are some.
For open source, try
www.codeplex.com, the Microsoft open source repository.
There are various "free" items there.
www.sourceforge.net is another good
source. If you go open source, please give back to the community if you
create enhancements or find bugs. That is what makes it work.