What is a (rough) rule of thumb for calculating fuel consumption in a modern jet airliner like Boeing 737 - 300?
Here are some very rough values for the 737-300, that a certain airline uses to compute manual flight plans in case the computer system fails.
Climb - 2950lbs / 15mins
Cruise - 5500lbs/hr
Descent - 600lbs / 20mins
Hold - 2650 / 30mins
Alternate - 1950 / 20mins
Min arrival fuel - 6300lbs
Recomended arrival fuel - 8300lbs
These numbers were based on a 27000lbs payload (approx 130pax plus bags)

source: http://www.airliners.net/forum/viewtopic.php?t=722273

[EDIT: I've edited the answer to remove any mention of the 737-300 being one of the most common 737 variants, thanks for the constructive comments!]