Update: Thanks to Dan for cluing me in to a hidden feature already built into Dashboard that lets you put widgets on the desktop. See the comments.
Mesa Dynamics Amnesty Widget Browser lets you use Mac OS X 10.4 Tiger Dashboard widgets directly on your desktop. It will even let you run many widgets on Mac OS X 10.3 Panther.
This idea is so obvious, I've been considering trying to implement it myself, just to see if it was easy. Mesa Dynamics is charging $19.95 for it.
(Mesa is using enough trademark symbols on its site that it's possible they're mistaking copyright law for trademark law: copyrights are automatic, but trademarks must be registered. It's also possible they're registering all of those marks, at $325 a pop...)
I'm not sure I completely understand the desired goal (only use a Mac for testing purposes at work, and Panther at that) but I believe there's a way to do this without the third-party app. See:
http://metatalk.metafilter.com/mefi/9714#227314