Apple created a dynamic binary translator called Rosetta and bundled it into their OS X operating system. As of OS 10.7, however, Rosetta was removed, and PowerPC applications are no longer supported on Intel-based boxes running the latest OS X software. Is there a way to bring Rosetta back? How can I run PowerPC applications on an Intel Mac?
You can "bring Rosetta back" by installing an older version of Mac OS X which supports it — Tiger, Leopard or Snow Leopard (on the latter it's an optional component). Rosetta was removed from later version for licensing reasons.
If you have an installation CD for PowerPC Mac OS X, you can use a full-system emulator to run it. Currently it seems your best bet is QEMU. If your application works on MacOS 9, and you have the appropriate installation media, you could also try SheepShaver (which emulates PowerPC Macs but doesn't support Mac OS X). PearPC is another PowerPC system emulator but I'm not sure how well it works (compared to QEMU).
As far as I know, there's no way to bring Rosetta back - so your only option (providing you have OS X install discs with PPC support), would be to set up PearPC - which emulates an entire system.
That being said, I don't know how well it runs - I only played with an 0.1 a very long time ago, and none of the more recent versions.
Assuming that your host computer has an Intel CPU and you wanted the most recent version of Rosetta, then you could run Snow Leopard 10.6.8 on VirtualBox. Unfortunately, just having Rosetta may not solve all your problems because some PowerPC applications or games require the classic Mac environment. In this case, you would need the classic MacOS 9 environment running on PowerPC. This can be done with QEMU or some other type of CPU emulator if your host computer is an Intel-based CPU.
Parallels and VirtualBox allow you to do whole system emulation as well, and running Mac on Mac seems a normal thing to do with it. These are much quicker than QEMU or PearPC.