heres a list from the scroller on the homepage of dotnetnuke.com

Heineken
British Telecom
Microsoft
Midas
Hertz
Nokia
Restoration Hardware
Champps
Cincinnati Bell
Bell Canada
Croatia Airlines
Budget
Alitalia
Allied
FMC
Progressive
HP
HRG
Kenwood
Schlumberger
Kodak
TVGuide
Lord
Manatron
Philips
AFL
Texas Instruments
TMNG Global
Pioneer
Sharp
Siemens
Mitsubishi
ExactTarget
F5
FastSigns
NASA
Sears
Merrill Lynch
Norfolk-Southern
Southern Copper
Syrian Air
Toshiba
Trammell-Crow
 

In addition you can view the showcase http://www.dotnetnuke.com/Products/Ne...

Finally, you can always use google - whilst this search isn't exact, it can help detect relevant sites e.g.

http://www.google.co.uk/search?hl=en&...= mostly returns .co.uk based DNN sites

and

http://www.google.co.uk/search?hl=en&...= mostly returns .gov DNN sites

Cathal

What are the issues that you're needing to justify it on?

The longer I've used DNN, the more powerful and flexible I see it being.  The team does a great job on the core / core modules, but it really does start to become great when you develop for your custom needs.

So to justify it, really I think it may help to look at the issues why you need to and address those.