Parallels Desktop 9 for Mac lets you seamlessly run Windows and Mac applications side-by-sid

OS 9 emulator works in Mavericks

Two readers report success with running SheepShaver/Chubby Bunny OS 9 emulation software in OS X Mavericks. The bundle (preconfigured with OS 9) enables the old pre-Intel Mac OS 9 ("Classic") applications to run on OS X Intel Macs. The current build was updated for OS X Mountain Lion last fall by Jon Gardner.

Fabrice Patisson in France says that the only issue is that his MacBook Air's fan runs a lot:

This is just to let you know that I downloaded Gardener's last bundle from the link on your page and that it works perfectly on my MacBook Air with Mavericks 10.9.2. I often have to run old thermochemical software named TAPP, which has no current equivalent, and I'm really glad to be able to do that. Thanks a lot.

I just noticed that when running Classic the fan of the computer systematically runs, a bit strange.

(Here is a direct download link from Jon Gardner.)

For Saumil Patel is happy with compatibility on Mavericks, though he hit a few snags:

Just installed it on OS 10.9.2 and works just fine. I have so many data files from years of research stored using applications that only work in OS 9. This will ensure that all those files are not lost. Plus, I still love Cricket Graph; nothing even comes close. Even Microsoft Word and PowerPoint files created by older versions don't open anymore in new Office programs. It is pathetic. Still trying to share a folder on OS X with the emulator but am having trouble mounting anything other than the shared folder.

Was not able to install PowerFPU and at times Office products (specially Powerpoint 4) crash unexpectedly. But Cricket Graph runs well and cut and paste across OS X and OS 9 seems to work well too.

If you've running Classic apps on your modern Mac .

For more reports on SheepShaver/Chubby Bunny, see our page Running Mac OS 9 apps on Intel Macs.