Many of us would have not forgotten the good old days of DOS. We use to play a lot while learning BASIC and Foxpro kind of stuff. Here is a way to play those DOS games in Ubuntu also. You need DOS emulator called DosBox. Let me brief how to do it.
Step-1: Install the DosBox as given below in terminal.
$ sudo apt-get install dosbox
Step-2: Get some DOS games downloaded from internet (Google will get you a lot of freeware games) and copy them to ~/dos_games folder (dos_games folder in your home directory).
Step-3: Run the DOS emulator as given below in terminal.
A new DOS window will open.
Step-4: Now you mount your dos_games folder as C: drive in DOS command prompt.
Z:\> mount C ~/dos_games
Now you should find your downloaded DOS games in C:\ path. Run those executables and enjoy!
Here is how a form of PacMan runs on my Ubuntu 10.04.