What is the speed of a standard airliner during the different phases of the flight?
Ballpark:


At the gate, 0 knots 
Taxi, 5-30 knots (usually 10-20) 
Takeoff, 150 knots (rough ballpark) 
Climbout below 10,000', 250 knots
Climbout above 10,000', 270-320 knots 
Climbout above 28,000', Mach .78 (ballpark) 
Cruise, Mach .78 (groundspeed ~450 knots +/- wind) 
Descent above 28,000', Mach .78 
Descent 28k to 10k, 280 knots (up to 330 knots, if ATC requests) 
Descent below 10k, 250 knots 
Traffic pattern,200 knots 
Final approach, 140 knots 
Taxi in, 10-30 knots
At the gate, 0 knots


Edit: these are fairly typical 737 numbers; other aircraft will have differences. Regional jets will typically be slower; 747 and 777 will often be faster (especially for takeoff and cruise).