My clear winner for favorite hack is using
CellCentric to get CNAM and 911/e911 (and 933 read back address check).
Second best, enabled by implementing the above, is using
GV's audio Caller ID and LisentIn on home phones (important point, if you do not do the above you will never get this feature for calls received via the Obi) for people I don't necessarily want to talk to. For these people, create a group 'Bad People' (or whatever) and set Call Screening to on.
Next is forcing call screening for unknown or private calls. Since you can do this on any other forwarding phone number, the hack part is being able to do this on incoming calls received via the Obi to your home phone.
Since you called this hacks and not 'regular features', using GV's ability to 'block' telemarketing calls, and have GV play the 'this line is disconnected or not in service tones and announcement' doesn't count, bit it's a powerful thing.