VirtualBox
Although I have used VMWare products for more than a decade, I decided to jump from that band wagon. It seemed that when ever Ubuntu changed (version upgrade etc.) VMWare Player stopped working and you could not install it. This became so annoying that I decided to start using VirtualBox (and qemu). Only negative thing in this is, that VirtualBox does not run Windows 98 virtual machine well while VMWare Player could. This is a small price, however.
Besides, VirtualBox has certain advantages over VMWare, which exceeds VirtualBox only in network configuration aspects (and these are only important in some enterprise level server virtualization). VirtualBox can handle USB devices directly, more easily than VMWare. VirtualBox can handle and switch between ISO disk images easily. VirtualBox can output sound through OSS, ALSA and Pulse Audio, while VMWare is limited to OSS.
VirtualBox is in Ubuntu repositories, but I advise not to use that. Go to VirtualBox www-site instead, and follow their instructions on how to add necessary repository and then install VirtualBox-4.0. You have to add one of the following lines into /etc/apt/sources.list. Choose the one which corresponds your Ubuntu installation.
deb http://download.virtualbox.org/virtualbox/debian natty contrib
deb http://download.virtualbox.org/virtualbox/debian maverick contrib non-free
deb http://download.virtualbox.org/virtualbox/debian lucid contrib non-free
Then get the keys, update apt, and install virtualbox-4.0
wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox.asc -O- | sudo apt-key add -
sudo apt-get update
sudo apt-get install virtualbox-4.0
Installation procedure tells you about vboxusers group, but it can easily slip unnoticed. So, in order to use VirtualBox, you must add at least yourself to vboxusers group (and anyone else who should be able to access VirtualBox). You can do this with administration tools, editing manually /etc/group file, or with command
sudo usermod -a -G vboxusers your_user_name
Extension Pack
I really recomment installing VirtualBox Extension Pack. You can find it in the Downloads page. Just download it and then issue command
VBoxManage extpack install filename
This will gibe you access to USB.
TeknoMagus 
Can you run MS-DOS and old DOS games in Virtualbox? I have some old games I would like to play.
Basically yes you can. But I do not recommend it.
Much better idea is to use DOSBox ( http://www.dosbox.com/ ) which is easy to install and configure. They also have excellent database to see which games work.
Important thing is to understand, that old DOS games take usually all processor power there is available. Unless you change the parameters, a game may run too fast.