Crunches are for fags and don't help for a variety of reasons. Those reasons have to deal with hypertrophy only being possible within a certain (smaller) range of repetitions and the range of motion on a crunch being fucktarded.
The best core lifts are the squat and deadlift. The deadlift is my very favorite lift and is deceptively simple. Really nothing can complete replace the squat and deadlift, and even if something could, it would involve 4+ lifts to accomplish what one lift would do for you instead. It's worth mentioning that my chiropractor has forbade me from doing squats for the time being, so I have had to work around it myself.
To get started without a gym membership or any equipment (ok maybe a doorframe chinup bar), check out convict conditioning. Here's an excerpt:
pushups and handstands use your core as a stabilizer, squats and bridges will directly work the core. btw using weights is still much better, but if you're itching to do something right this very moment that picture can be helpful.