Configure so that the space bar works both as space and control; when the space is pressed alone, as a space, and pressed with others, as ctrl. So Space + x is treated as Ctrl + x.
You can do it with AutoHotKey in Windows, and with "at-home-modifier" in Xorg in Linux. (X, but Linux only.) You can use Karabiner (formaly known as keyremap4macbook) in Mac. (In fact, I am the author of at-home-modifier. =)
You can do more, if you have a keyboard with many keys around the space, like Japanese keyboards:Japanese keyboard http://www.owltech.co.jp/products/keyboard/KB86STD/KB86STD_B-320.jpg
My bottom row is basically EscBSSpaceEnterTab, but when used as modifiers, it's AltShiftCtrlShiftAlt. (For example, if you hold down Esc first and then Space, it's Alt+Space, but Space followed by Esc, it's Ctrl+Esc. If you press Space, Esc, and x, then it's ctrl + alt + x.) All can be pressed with the thumbs. You can order a Japanese keyboard from say amazon.com. You don't have to speak Japanese.
This is extremely handy. For firefox (sorry, not emacs) for example, focus a link, and press Ctrl + Enter; then it'll be opened in a new tab. (By also pressing Shift, it'll switch to the new tab, rather than staying on the current.)
(The above picture is the one the author of at-home-modifier uses. The maker doesn't sell this model any more, though.)