Someone mentioned bicycle parking; I don't use it but our building just added that.
Dog-friendly office (mixed bag, they're nice to have around but they can be loud and at least one is not fully house trained).
Decent health benefits (crucial).
Pre-tax transit benefits.
401K with employer match (after the last couple of jobs I now consider this crucial and regret not requiring it during previous searches).
Dinner and a cab home if you have to stay exceptionally late (almost never take advantage, but highly appreciated when leaving the office at 9:30pm during a snowstorm).
Volunteer day when the entire office turns out once a year to work with a local nonprofit. (Love this! And I think it does more for employee solidarity than any social event.)
Unmetered PTO that at least in some departments we're actively encouraged to take advantage of. Awesome if it makes you comfortable taking off more than a couple weeks a year. Awful if it makes you uncomfortable taking any time off.
Paid parental leave. (One of those things that should be universal and we're doing better at than most places but could still offer more.)
Flexible hours.
Summer half-day fridays (best thing ever!)
Beer fridge (with the general expectation that it'll be opened only on Friday evenings; rarely used)
Office parties (as an introvert, I loathe these, but I guess some people enjoy them).
Wish we had:
Private offices, or at least cubicles. Or noise dampening treatments for the concrete ceiling or floor. Or something.
Non-fluorescent lighting.
Health initiatives.
I cannot think in anything that I wish I had.
A part startup I worked for (10 employees at the time) installed a shower for employees.