Windows only: Manufacturers make a lot of claims about laptop batteries nowadays, but it's hard to tell what your battery's true life is. Battery Eater will test it at load and under normal usage, so you have realistic expectations.
Battery Eater is a simple program that monitors your battery as it runs out of juice. The default test is a full load test, which will tax your CPU and GPU until the battery dies so you know your absolute minimum battery life. You can also run a "reader test", which merely displays and scrolls through text on your screen until the battery dies, so you know how long your battery will last through more typical use. Obviously, you want to start running the test after your battery has fully charged to get accurate results.
When you plug your computer back in and relaunch Battery Eater, it will display a results graph of the last test, letting you know how long the battery lasted, and what you can generally expect from it in the future. It's a helpful app to try out, albeit shocking if your expectations are significantly different than reality. If you're unhappy with the results, you can always make sure you're doing everything to maximise your laptop's battery life, too.
Battery Eater is a free download for Windows only.