Anybody who has the cash and want to do it. Governments seem pretty realistic. Of course, terraforming (unless it's some magic trick) is a long process, so convincing them to throw money at it for a result they won't be around to see is the hard part. Once you have set it in motion, it gets pretty easy, depending on the planet.
So, there's this planet with a bunch of Unobtanium, and you of course want it, but the planet has no air to breathe so it's going to be a pain in the ass to mine the Unobtanium. Easy solution (in fiction): bring some goddamn trees. Problem solved. Then the trees just do their tree thing and reproduce, eventually covering the whole planet. (that would take a helluva long time though)
It's never that easy in reality, of course: the trees can't live in a hostile environment any more than people. If the fictional example is sliding more towards the hard sci-fi area, the trees will be introduced very late in the process, with less complex organisms getting a foothold first.
If the culture in question has age-halting medical technology, an investor might be able to live long enough to see the return on an investment in terraforming.
A culture might regard terraforming as a religious or ideological obligation.
If your home planet is going down the drain, you might have a motive to create some spares. Of course, you'd want to spend similar efforts on repairing your home planet, but it might be too far gone, or the efforts might not work and you'd want to hedge your bets, or terraforming efforts might be much trickier with billions of people in the way.
I'll second the "religious and/or ideological" obligation. I could easily see a major terraforming effort (such as terraforming Mars) becoming a quasi-religious movement for the people living on Mars and involved in the process.
I question the viability of Venus as a terraforming candidate. Aside from the crushing atmosphere that you would have to remove somehow, the planet is inside the inner boundary of the Sun's Habitable Zone, making it perpetually prone to a runaway greenhouse effect even once terraformed. Did I also mention that the planet suffers from gigantic outbursts of volcanic eruptions that effectively re-shape its surface every few hundred million years? One of the downsides of Venus having no plate tectonics.
Here you go: http://en.wikipedia.org/wiki/Terraforming_of_Venus The first step would probably involve positioning LOTS of solar power collection satellites around the planet. Solar energy gets beamed in the form of microwave radiation to Earth, and less solar radiation reaches Venus, cooling it down. The terraforming process would pay for itself even before you get the enormous payoff of useable land. The next step involves converting elements in its atmosphere to solid or liquid form (the wikipedia article goes into a lot more detail) and the introduction of more water, probably by redirecting comets to a collision course. The incredibly long day/night cycle can be handled with large orbital mirrors that rotate to control how much light makes it to the surface (the wiki article goes into more detail here too). The vulcanism issue can probably be solved by introducing a sufficient amount of water to the planet's surface (most likely brought in from comets). Water seeps into the crust and makes it more viscous, allowing heat from the core to escape at plate boundaries just like it does on Earth, instead of building up under the surface until it explodes.