VirtualBox (available in Debian as virtualbox-ose with a m-a-buildable kernel driver: virtualbox-ose-source), is quite interesting, and works great on both sid and lenny. [Why OSE? Open Source Edition.]

Example: X freezes sometimes when gtk-2.11 is installed, maybe due to bugs like xfwm4's Debian bug #442053, which is quite annoying since epiphany's trunk needs it. Then develop in a virtual machine and restart it when needed, leaving the host system unaffected. It is also quite easy to duplicate, pause, resume, etc. the virtual machines.

Note: Virtual machines are stored in .vdi files (Virtual Disc Images). Copying them isn't the way to clone images, since a UID is stored inside them. Use vboxmanage clone original.vdi clone.vdi instead.

An annoying bug in VirtualBox is Debian bug #443500: the AltGr key isn't passed to the virtual machines. Fortunately, upstream's changeset 3939 fixes that.

Now {, }, [, ], @, |, # are available! \o/