Neil,

It might help if you mention which program? =)
I think I stumbled over that link a while ago on the SL forums though. *googles*

I found it, it's by Johan Durant and you can download the SL Clothing Previewer here for Windows and here for Mac. The corresponding SL forum post can be found here. You do need an avatar with payment data on file to enter those.

Avpainter is fun too - although can't beat Photoshop - but not free.