How much Bitcoin will I mine right now with hardware X? Say I have a given piece of hardware (CPU/GPU). Where can I find out how much Bitcoin will I mine per day?
Go to http://bitminter.com/test and click the "test start" button. If you have Java installed the miner should launch. Click "engine start" on your GPU(s) to start mining and the GUI will show how many bitcoins per day you will make (on average).

Note that you are actively mining in a pool without getting paid. This test page launches a version of the miner which is only meant to be used for a short time for testing.

Update for 2015: CPU and GPU mining are both long dead. When you buy ASIC mining hardware you will know its hashrate before you buy. Don't buy if you don't. Google "bitcoin mining calculator", input your hashrate, and it will tell you how much you can earn right now, on average. Note that the difficulty will change in the future so your earnings will not stay the same. Also note that with most pools actual earnings vary with luck. Finally, avoid the typical newbie mistake of confusing TH/s and GH/s. 1 TH/s = 1000 GH/s.