Linux: We've discussed several apps for Windows and a few others for Mac that remind you to take breaks periodically, but Linux users need some love too. DrWright, a free, open-source utility that locks your system periodically so you take a break, gets the job done.
The utility is simple, but works fabulously: open the app's settings, and tell DrWright (referred to as "Typing Break" after install) how long you want to go between breaks, and how long you want your break to last. Give yourself the option to postpone those breaks, or leave the "Allow postponing of breaks" checkbox empty to give DrWright complete control and force you to stop working. When it's time for a break, the app will lock your screen for the duration of your break, and it won't let you back in until your break is over.
Sure, you could find creative ways around the block, but you really should get up, move around and take a break from typing regularly. There are packages for Ubuntu, Arch, Fedora and other distributions at the link below.