How do I get Wi-Fi info from within Terminal? Is there a way to get BSSID and the list of available Wi-Fi networks from within Terminal?

I.e. I'm looking for something similar to ifconfig run0 scan on OpenBSD, which lists all access points, BSSID, signal strength etc.
There's an airport utility buried that you'll want to use. Create a symbolic link in /usr/local/bin to the utility for quick access with this command:

sudo ln -s /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport /usr/local/bin/airport

Now, you should be able to run airport -s and get a list of available networks with BSSIDs.

If you hold the option key while opening the WiFi networks in the Menu Bar, you can also see the BSSIDs one at a time, although it seems like you wanted the CLI-like version from OpenBSD.

UPDATE:

"rootless" AKA System Integrity Protection (SIP) has been added to OSX since I wrote this answer originally, which makes /usr/bin and /usr/sbin read-only. It is proper to use /usr/local/bin, so I have updated the answer so that the symbolic link is created there.