Running on old hardware is a key theme for me, I've got several old laptops lying around, and I also run copies of Linux in a virtual machine, either on VMware Player or on VMware ESXi. In either case, I've got limited memory and disk space, so I'd prefer the distribution not to require much of either.
Lubuntu, the lightweight Ubuntu-based distribution that uses LXDE as its default desktop, fills this niche very well. It's a solid distribution, uses a fairly minimal amount of RAM and disk, and although LXDE lacks some of the visual pop that some other desktops have, it's still highly customizable and good looking. You can download Lubuntu at http://lubuntu.net/
Xubuntu is another variant, this time with the XFCE desktop which is a very popular desktop environment, good looking and customizable. Xubuntu can be downloaded at http://xubuntu.org/
Kubuntu is yet another, that uses the snazzy KDE desktop by default. Talk about eye candy, KDE takes the cake in terms of cool effects. Kubuntu can be downloaded at http://www.kubuntu.org/
There are more, Ubuntu is used as a basis for many Linux distributions, each with a different justification for its existence, whether it be different default desktops, different default office applications, web browsers, applications that cater to scientific, multimedia, ease of use, the list goes on and on. For a longer list of Ubuntu-based distros, and tons of other Linux and Unix-based operating systems, check out distrowatch.com and use the search link at the top of their page to search for Ubuntu-based distros.