Tab control (and bunch of other) comes with AJAX control toolkit.
See http://asp.net/ajax/
Samples: http://asp.net/ajax/ajaxcontroltoolkit/samples/

George.

There are loads:
http://www.google.co.uk/search?sourceid=navclient&hl=en-GB&ie=UTF-8&rlz=1T4GZEZ_
en-GBGB252GB252&q=%22ASP%2eNET%22+tab+control+free

However, beware of those which do most of their work server-side, especially
the AJAX ones, as they can't support client-side cross-tab validation...

E.g. if you need to validate the contents of a TextBox in the first tab
against the contents of a TextBox in another tab, you won't be able to do it
without a round-trip to the server whether via AJAX callback or full
postback...

I got so frustrated with this that I ended up using this, though I made some
fairly extensive mods to it:
http://www.barelyfitz.com/projects/tabber/

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.

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I've indexed a bunch of useful free and useful controls here:
http://SteveOrr.net/FreeControls