tdf.org ($35-40/year) - the Theatre Development Fund provides many discount tickets for tons of shows all over nyc. they have eligibility requirements, but they're quite generous with 'non-exempt employee' and 'under 30 years old' as some of the possible criteria for membership. tdf is most famous for their red TKTS booth in Times Square, where you can get same day discount tickets in person to many shows.

willcallclub.com ($20-50/year) - i've seen some great shows via this discreet ticket club that provides nearly tickets for shows that need a boost, sometimes called 'papering the house', according to their site. they have a ton of club rules, the most important being 'don't talk about the club'. read the rules carefully!

the following programs are all totally free! just navigate to their sites to sign up.

linctix (free!) - the Lincoln Center Theatre's own discount ticketing program, available for theatregoers ages 21-35. tickets are $32 to every show at the Lincoln Center Theatres.

hiptix (free!) - the Roundabout Theatre Company's discount ticketing program, available for theatregoers ages 18-40. tickets are $30 to every show produced by Roundabout.

2st.com (free!) - 2nd Stage productions have a discount code for theatregoers 30 and under. enter '30UNDER30' and you'll get access to $30 tickets to any of the productions by 2nd Stage. the seats available for the discount do vary, but i've heard of folks getting decent seats with the discount.

mtc's 30 under 35 (free!) - Manhattan Theatre Club's discount ticketing program, available for theatregoers 35 and under. as a perk of the program, they usually save some of the best seats for the folks that get these tickets!