Sethbling made a tutorial for his volcano: http://www.youtube.com/watch?v=rOHnWS8pdSg

It's very easy to do it.
Change the direction of the animated flowing lava texture upside down? Or something like that?

Like Sed said(lol Sed said) SethBling made an erupting volcano with MCEdit, which will be playable in vanilla(probably your best option). You could also build your volcano upside down and then the lava can only flow up Or use dispensers facing upwards, or have dispensers floating above the volcano and then make them use a lava bucket and then use tnt to blow the dispensers up.

OH!!!! I have an Idea! (But it won't be very high) set a dispenser and place lava in it

You can make a Volcano with MCEdit and filters by selecting falling sand then changing its ID to Lava's ID number, then create spawners for that Entity so they can shoot up into the air.

This is from SethBling's Mount Dinnerbone map, and a map I made inspired by that.


Anyone will be able to use your map, MCEdit is a program that let's you build stuff in the game and other things without mods.

Lol maybe a mod or something?

not really possible, it sounds like you want it to erupt?
if not, if you just want a togleable flow, then have pistons set up around the edges to lower and let the pool of lava loose?

That's all i got

There is no way without mods, but that would be cool.