Quantum Mechanics
My scientific passion used to be chemistry but it has since shifted to one level smaller: quantum mechanics once I got to graduate school, particularly the work of Bohr and Heisenberg on complementarity and uncertainty, respectively. Quantum mechanics basically studies subatomic particles and their make-up.
What guys like Einstein figured out at the turn of the century is that the laws of Newtonian mechanics do not apply on the subatomic level. The fundamental building blocks of reality follow counterintuitive, alien logic that defies common sense, which is what makes them so cool.
What actually got me first interested in quantum mechanics was Prof. Edward Birack's (Victor Wong, of Big Trouble in LIttle China fame) lecture to his students in quantum mechanics from John Carpenter's masterpiece, Prince of Darkness (1987):
"Let's talk about our beliefs, and what we can learn about them. We believe nature is solid, and time a constant. Matter has substance and time a direction. There is truth in flesh and the solid ground. The wind may be invisible, but it's real. Smoke, fire, water, light - they're different! Not as to stone or steel, but they're tangible. And we assume time is narrow because it is as a clock - one second is one second for everyone! Cause precedes effect - fruit rots, water flows downstream. We're born, we age, we die. The reverse NEVER happens... None of this is true! Say goodbye to classical reality, because our logic collapses on the subatomic level... into ghosts and shadows. From Job's friends insisting that the good are rewarded and the wicked punished, to the scientists of the 1930's proving to their horror the theorem that not everything can be proved, we've sought to impose order on the universe. But we've discovered something very surprising: while order does exist in the universe, it is not at all what we had in mind."
Who wouldn't want to learn more about this?
A very good Wiki site designed as a non-technical introduction to QM: http://en.wikipedia.org/wiki/Introduction_to_quantum_mechanics.