So this is an actual "hack" and not a fully fledged app but it allows
games that require 640x480 to run on Color Capable macintoshes with a
12" RGB resolution (512x384) resolution. The idea behind it came as
alternate solution to get more games to run on my Color Classic II,
without having to do the 640x480 HiRes (or regular VGA) mod to it.
There are quite a few games out there (eg: Duke Nukem 3D, Ishar 3,
Marathon) that don't start if the user doesn't have a VGA compatible
monitor but allow you to downscale their resolution (320x240) or
reduce the drawing area so they should technically be playable on less
than 640x480 resolution.

Tutorial:
\- Run fVGA.0.1
\- Press CMD+1 to set the main device (screen) rect and pixmap rect to
640x480
\- Run the game you want to run
\- From the Multitasking Menu switch bag to fVGA.0.1
\- Press CMD+2 to clear the settings and quit fVGA_0.1;
\- Play your desired game.

So far I've tested it with:
\- Duke Nukem 3D (the reason I wrote this hack)
\- Marathon 2 (works pretty well)
\- Warcraft 3 (a bit hard to navigate but playable)
\- Heroes 1

More testing needs to be done, but I'm upping the first version and
its source in case anyone else wants to try it. Tested on Mac OS 8.1
on a Color Classic II equipped with a Sonnet Presto Plus (040).

I recommend using it with a init/cdev that allows you to switch
between applications using a key combination (like the amazing Option-
Tab Program Switcher), just in case the game you're trying to run
doesn't allow you to access the Multitasking button or, you know,
something goes wrong and you need to focus on the app to quit it (CMD
+ 2).

Edit: Here's [a video][1] showing fVGA in action on my CCII.

Compatibility
Architecture: 68k

Color Classic, Color Classic II, Any Mac outputting to a Apple 12" RGB
Monitor.

  [1]: https://www.youtube.com/watch?v=4LqCGkjCwiE