easiest thing to do is google the app you're interested in evaluating and see if there's a Mac version available. This should also give you some idea of the size and name to expect. An indirect list of popular mac apps is Serial Box (usually updated the first of each month; e.g. Serial Box 8.2003). The other advantage of searching for Serial Box is that it's mostly for OSX, so the chances machines sharing it are Macs.

.dmg files are the newer way of sharing mac apps over the internet: they are pretty much disk image files for OSX, so that's another way to search. For OS 9, search for disk images with .img, or .img.sit. (some .img files are pictures images though).

I use searches for serial box to test connections, or just to find hosts to browse.

If your OS thinks a program is a document, the program is probably not much use with your set-up. You may want to save those files and check them the next time you boot into the other OS.

Cheers. If you find something that works, buy it and support Mac developers. Hope this helps you avoid downloading stuff you can't use.