Ubuntu is still the most popular Linux distribution around, but a lot of people are leaving it for greener, Unity-free pastures. If you like Ubuntu but don't like its interface and default apps, Ubuntu Builder can help you create your own, customised version of Ubuntu that you can install on any machine.
Ubuntu Builder is very simple to use. Just fire it up in any version of Linux, give it an Ubuntu ISO to tweak, and start making your changes. You can choose from a number of alternative desktop environments (like GNOME, LXDE, KDE and others), as well as add your favourite packages so they'll be installed along with the OS itself. You can also add other sources to synaptic, give your distribution a name, and more. When you're done, just build the ISO and you can install your custom version of Ubuntu on any computer. It isn't quite as powerful as something like SUSE Studio, but it's great for Ubuntu fans that just want something a bit more tweaked to their liking.
Ubuntu Builder is a free download for Linux only. If you're looking for something similar on Windows, check out RT Se7en Lite, which lets you build your own custom version of Windows 7.