First I want to make sure that the importance of proper hardware calibration is understood. Your eyes are not perfect(mine especially) and you can certainly give it a shot to determine the best brightness, tint, gamma, etc. but you are not going to be nearly as spot on as a dedicated piece of hardware. In reading you will find that as the display gets older, it becomes more out of sync with the correct values, and hardware calibration becomes more important. So even if you can't afford it now, down the road it is still a good investment.

So onwards to how to actually do a visual calibration. You are in luck, because OS X actually has a utility built in to do exactly what you want - Display Calibrator Assistant. To find this utility all you have to do is open up System Preferences, then the Displays app, click on the Colors Tab, and you will see an option for Calibrate. From that option, make sure you are in expert mode, follow the instructions, and you are on your way to visual monitor calibration bliss.

Other options exist that are not included with OS X. You may even receive one with your monitor as part of its software package. A popular option is a shareware program SuperCal.

If you are looking for a tutorial on how to use the built in Calibrate tool from Apple you can find that here.