QFT. My abs were dead for about 3 weeks when I first started doing backflips... maybe 20ish in the session. Makes me think I actually pulled an ab or something

Why would you need to train your core explosively in the first place? If your goal is to strengthen it for movements in parkour and free running, heavy lifting and tumbling (flips and whatnot) are the way to go.

I have read the first page and I'm telling the truth when I say that those are the most effective ways of developing core strength. You don't need to train your core explosively for movements that require explosive contraction of the core muscles. The explosiveness you require for doing tumbling will best come from doing tumbling, not from crunches or some accessory exercise. Doing the movements is all you need to do, and if you want to boost performance more, heavy lifting is the most cogent path, imo.

As for having no weights at the moment, that's no excuse. If you want to make big gains, weights are probably the easiest way to do it. You can make weights or buy them, but either way they are pretty much indispensable. Bodyweight exercises have their place, but lack of variability and adjustability means that beginners don't have access to the more effective bodyweight exercises. With weights, you can make constant and easily measurable progress. Don't get the wrong impression, though; I'm no weight fanatic. Right now I'm on an almost entirely bodyweight workout regiment.

You can easily make weights for free or for very little money, if you're clever about it. It just depends on how much you want to progress physically. If you're okay with slower progression and limited options, you can skip the weights, and that works for a lot of people. Once you have achieved the baseline level of strength to execute them, certain bodyweight exercises can be far more effective than weights, but then again, you're still limited. Most lower-body exercises, for example, cannot be made intense enough for strength gains after a certain point. Pullups and dips also come to mind.

The real obstacle here is yourself. If you consign yourself to thinking that there's no way for you to get weights because you don't really have the money to spend on them, you're never going to get them and you will have missed that opportunity. Like I said before, if you put your mind to it, it's not all that difficult to find cheap or used weights, or to just make some.

They also have these crazy little beasties called boulders

1.) They're free
2.) They're easy to find
3.) They typically work your grip strength more than a barbell
4.) Chicks dig guys who pick up rocks

... im a girl o.O

And Conversely, Guys dig Chicks who pick up rocks... A guy likes a capable woman. Very much