there's also the TKTS booth at the SouthStreet Seaport (on John Street -- a little hidden though) that's like the booth at the Marriott (Times Square) but the tickets go on sale earlier in the day and also the matinee tickets go on sale the day before. Just so you know the tickets usually don't come in at 50% -- some are 50% some as low as a 25% discount. Usually the seats are somewhat on side or back.. may be slightly obstructed -- they're what's left over actually. You might also want to look into "rush" tickets that go on sale the day of show that depending on show may run around $25 each.. there's also lottery tickets (know that Spelling Bee, Drowsy Chaperone, Chorus Line, Hairspray and Wicked offer these tickets -- you get front 2 rows (don't get choice of seats.. they go in row when you line up) where you put your name in for a drawing of these 'covetted <and cheap> seats... Usually it's cash only and 2 ticket max per person. It's luck of draw but on 3 times our family has been very lucky and has seen Wicked (in August), Spelling Bee (cute show), Hairspray (we all saw it day after Thanksgiving.. front row dead center) and Chorus Line and Drowsy Chaperone (March) this past year.
Good luck. It's a way of our family getting to enjoy what NYC has to offer while watching the wallet. Now my DD (age 11) watches to see what new shows will be offering lottery. Luckily we have a place to stay in NYC cheap (camping out in Aunt's apt. in lower Manhattan).
Good Luck
Carolyh