What tools are there for measuring Macbook wireless strength?
If you're just looking for a quick signal strength number, option-click on the AirPort icon in the menu bar. Under the connected network you will see several pieces of information:



RSSI is your signal strengh in dB. Higher (closer to 0) is better.

If you're looking for noise or the signal strength for multiple access points, I suggest checking out iStumbler: