FotoStation from Fotoware (www.fotostation.com) is designed specifically for photo organization. Its 4.5 version runs on Mac system software from OS 8.0 through OS 9.2. It will run in "Classic mode" under OS X. iView Multimedia's Media Pro 2 (www.iview-multimedia.com) offers a complete set of image-management tools and even includes a slideshow program feature.
A more advanced system is Portfolio from Extensis (www.extensis.com). Extensis has developed products for the professional imaging industry; Portfolio is a very powerful organization program, but it's also harder to use. It has a Mac version that runs on OS 8.6 through OS X.
Lastly, you could consider upgrading just your operating system to OS X. OS X comes with iPhoto, which will allow you to organize and store your photos. Upgrading to OS X is certainly a big step (like going from Windows 3.1 to XP for you Windows users out there), but it might increase your options for other software applications that you can use on your iMac.
And remember—it's PCPhoto, not WindowsPhoto!