Lots of Mac applications provide guidelines when you are trying to lay things out. You'll find them in the XCode interface builder, iWork applications, and many graphics applications like Photoshop. While the guidelines are very helpful, there are times when they can keep you from placing something exactly where you want it since the placement tends to "snap" to the guidelines. If you want to disable the guidelines on the Mac for your current operation only, hold down the Command key on the keyboard. Let go of the key and the guidelines will come back. I find this shortcut very useful when trying to make minor tweaks to layouts.

Another way you can usually skip the guidelines, though it depends on the app, is to let the placement happen on the guideline. With the object still selected, you can then many times use the cursor keys to move the object 1 pixel at a time.