[Phantasm]

I agree with nanoboy, and highly recommend several others: Power-Ups 1: Imbuements, if you wish to have someone like Gambit who can charge up anything, or Yondu who can make arrows change course in mid-flight; Power-Ups 2: Perks, for a snotload of flavorful little extras; Psionic Powers, for a bunch of premade easy-to-insert abilities and several new modifiers; and Martial Arts, for the expanded combat options. The -Tech books are also handy for equipping your characters - or your super-team. I don't have them yet myself, but the Action! books are also highly recommended by others on the forums.

And, if your players are like my old gaming group, when you ask for "supers" they give you a party of half-demon sorceresses, genies, half-angels, leather-clad flaming skeleton bikers, and other supernatural entities, Magic and Thaumatology come in very handy.