DOSBox is a PC emulator with builtin DOS intended for running DOS Games primarily. The latest version 0.61 has already support for a lot of DOS games. DOSBox has extensive emulation for different kinds of hardware, such as GUS, VESA, IPX networking/modem connections over TCP. DOSBox is licensed under the GPL.
DOSBox emulates every CPU instruction, which gives it the advantage of running on non-i386 platforms. However, this also makes it much slower than i386-only emulators that use the CPU's compatability mode. On modern machines this is unlikely to be a problem, most DOS games were written for now ancient hardware.
Of course gaming is not the only thing what you can do with dosbox. You can run other dos exe or com programs also, use the norton commander and do batfile programing.
For Ubuntu, in this case 9.04, install dosbox by opening a console and entering:
apt-get install dosbox