I was looking for such an expansible, easy to use app, since a long time ago when I lost track of the app I was previously using.
I tried with gEDA suit and OpenCollector database, but no app inside them was such a cross platform, windows able, self containing app as Logisim.
(I couldn't even install the gEDA suit since my linux skills are quite poor)
I found Logisim almost by accident and I found it almost perfect for my needs.
I already submited a few feature requests, to try and help make this app even better.
I would like to submit it to Open Collector database so to impulse it and make it reach more people like myself if Carl agrees.
For reference, Open Collector database's url is http://opencollector.org/
Thanks! I'm glad you're finding it useful. And thanks for the feaure requests, too. (I agree that two of them would be nice, and there's a good chance they'll make it into a release of Logisim over this summer. I didn't feel like I understood the other one, concerning an unspecified input/output component.)
You can feel free to submit Logisim to Open Collector.