Apple - 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:

You can get quite a bit of info from the command line, with the (well-hidden) airport utility. With the -I flag it'll tell you about the current network:

$ /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I
     agrCtlRSSI: -43
     agrExtRSSI: 0
    agrCtlNoise: -96
    agrExtNoise: 0
          state: running
        op mode: station 
     lastTxRate: 130
        maxRate: 130
lastAssocStatus: 0
    802.11 auth: open
      link auth: wpa2-psk
          BSSID: 0:24:1:0a:42:93
           SSID: My Wireless
            MCS: 15
        channel: 5

...and -s will show more info about other visible networks:

$ /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -s
                            SSID BSSID             RSSI CHANNEL HT CC SECURITY (auth/unicast/group)
                     My Wireless 00:24:01:fa:42:93 -44  5       Y  -- WPA(PSK/TKIP,AES/TKIP) WPA2(PSK/TKIP,AES/TKIP) 
                          hobbit 00:15:05:19:8a:03 -86  7       N  -- WEP 
                     My Wireless 00:24:01:ef:91:ab -75  3       Y  -- WPA(PSK/AES,TKIP/TKIP) WPA2(PSK/AES,TKIP/TKIP)

It doesn't have a man page, but if you run it without any options it'll list its options.

System Profiler will list available Wireless Networks as well as the following information

NetworkName:
  PHY Mode: 802.11n
  BSSID:    c0:3f:e:df:1b:be
  Channel:  6
  Network Type: Infrastructure
  Security: WEP