How to escape the center of a room without gravity?

Since you have air around you, you can just take a deep breath and blow it out.

There's actually no need to turn you head while doing this, as some other answers suggest. Air has a high Reynolds number at human scales, and so the scallop theorem does not hold: even though the movements of inhaling and exhaling are reciprocal, the airflows they create are not.

(You can test this yourself by holding a hand in front of your mouth: you can easily feel the jet of air created by blowing out, even with your hand fully extended, but you can't produce a "reverse jet" no matter how hard you inhale.)

In practice, the momentum produced by inhaling is pretty negligible, as air flows in towards your mouth and nose from all sides, and so the only thing that matters is which way you exhale. By blowing air out of your mouth in one direction, you create a net airflow in that direction, and so, by conservation of momentum, propel yourself in the opposite direction. It works for squid and jellyfish (and scallops!), and it will work for you, too. Maybe not very efficiently, but surely enough to reach a wall in the tight confines of the ISS. Now, if we ever start building space stations with huge air-filled bubbles hundreds of meters across, then this might become a problem, but until then you should be fine.

Besides, you may not even need to resort to such huffing and puffing. Any real space station designed for human habitation in microgravity needs to have active air circulation fans anyway, both for heat distribution (important for both humans and equipment, since convection doesn't work in microgravity) and to keep exhaled air from accumulating around your body e.g. when you're sleeping. So in practice, the air around you will be moving slowly anyway, and you just need to wait until this ambient airflow pushes you close to a wall.

And of course, on the actual ISS, I doubt there's even any space big enough to properly pull off this prank. The largest open spaces on the ISS, like the Kibo pressurized module, are surrounded by ISPRs that are about 2 meters (6 ½ feet) wide, effectively making the interior cross-section a 2×2 meter square. Even if your crewmates somehow managed to position your body lengthwise along the center axis of an otherwise empty module so that you couldn't just reach out and grab a handhold, you'd just need to twist around like a cat (or, more likely, just flail around semi-randomly) until you managed to turn yourself 90° around, at which point either your toes or your hands should surely be able to reach a wall.

It is worth calculating (rather than just speculating) some of the methods described.

  1. Breathe in one way, and out the other. Your resting tidal volume is about 0.5 liter; take a deep breath and it can be 3-5 liters (thanks @Aaganrmu). That is about 4 gram of material. If you purse your lips to increase the velocity with which you expel the air, you can get it to around 10 m/s (I estimate this from the data in this paper which measured the velocity of air in a cough - 15 m/s - and spoken word - 4 m/s.). This gives a net momentum of 0.04 kg m/s, which means a 70 kg astronaut will get a reaction velocity of 0.05 mm/s, moving 1 cm every 20 seconds. But if you do this 10 times per minute, your acceleration will be about $10^{-4}~\rm{ m/s^2}$ and you will travel 2 m (to the nearest wall) in about 200 seconds. Feeling somewhat dizzy from the hyperventilation... Note that it's not necessary to turn your head: it's enough to breathe in slowly, and out rapidly.
  2. "swim" with your arms. If you can change the area of your arms by 20 cm$^2$ between the "forward" part of the stroke, and the "back" part of the stroke, and you can move the hand with a peak velocity of about 2 m/s for 50 cm, the approximate drag force will be $F = \frac12 \rho v^2 A C_D = 0.5*1*4*0.02*1.0 = 0.04~\rm{ N}$ - four times more than breathing out. And of course you can probably move your arms a great deal faster - let's say one complete stroke (two arms) per second, for an acceleration of $6\cdot 10^{-4}~\rm{m/s^2}$ and a time across the capsule of 80 seconds.

Combining the two techniques, the exercise with the arms will allow you to breathe more rapidly (at peak exercise, an adult male can move about 100 liters of air per minute - that is 10 times higher than the value I used). This should comfortably get you to the side of the capsule in under a minute.

If you can increase the mass you accelerate, you can greatly improve on these numbers. I briefly considered that spitting might be the answer, but your mouth will run dry pretty quickly. Other bodily fluids would greatly improve on the time - but given that you still have to live in that space after the fact, I think that spending a bit more time waving your arms about, then laugh at the videos your crew mates made, is the best approach here.

There are some obvious and boring solutions e.g. breathe in then turn your head 180 degrees and blow the air out. Repeat for long enough and you'll build up a net velocity.

More interesting is that if the gravitational field is not completely uniform you can actually swim in it. However this is such a small effect that you'd have died from old age (let along starvation) before you managed any significant change in position.