Buffets: $1 - $2 (depending on experience)
Drinks tab: $1 each (approx)
Meal tabs: minimum $2; 15% - $20 (strongly depending on experience) - when I've been very dissatisfied, I've rounded down to 12% or so,
and been known to leave somewhat over 20% if I've really clicked with the server.
Taxis: rounded up a couple of dollars ($11.60 = $14.00, etc.)
Shuttle Drivers: Again with the "clicking" deal, so $1 - $2 on occasion.
Barber: Usually $2/visit

I don't tip hotel housekeeping, nor folks who obtain a taxi for me by making a single phone call or rasing their hand to a waiting vehicle nearby.