I'd be wary of suggestions for non-combat adventures; if these guys are powergamers you'll just bore them. And a special encounter only works once.
My suggestions (some of which others have already suggested):
(1) Increase the EL of encounters. Someone said +2 which is a good place to start, but adjust upwards until things get challenging again. Be sure not to kill off the non-twinky characters in the crossfire! Don't award extra experience for the increased EL.
(2) Avoid single-monster encounters. Use multiple creatures whenever possible, and try to mix creatures to include a variety of abilities. Find ways for them to complement each other.
(3) Look at their specific powers, items, feats, classes, etc. to see what makes them so effective. Be sure to build your NPCs the same way, with similar equipment. Optimize NPCs, then go back and re-optimize to build on each other's strengths. After all PCs to this. Read the Sultans of Smack thread for inspiration.
(4) Watch their tactics and use the same tactics back at them. Every PC party I've seen works like a well-oiled machine: they are always ready for action, have preferred tactics that play to their strengths, and always go straight for the jugular: the best defense is to win initiative and kill the enemy in the first round. They sleep in their armor, for crying out loud! Make your NPCs behave exactly like the PCs: ruthless, suspicious, alert, prepared. Also note whether your PCs are selfish with equipment and spells, or whether they share and lend to each other. Do PCs kibbitz in combat, sharing OOC knowledge and coordinating attacks? Let your NPCs do the same. Do the PCs single out spellcasters, or coup-de-grace fallen foes in the middle of combat? What comes around goes around.
(5) NPCs aren't stupid, and if they see an effective PC party they'll not only copy them but will also prepare a defense against them. Let their reputation precede them. Forewarned is forearmed.
(6) Don't let them choose their battles all the time. PCs love to catch NPCs napping, sneak up, and set traps for them. Wait until they think they're safe in between adventures, the wizard has loaded up on identify spells or is busy making magic items and everyone else is off guard, then have their enemies catch up and attack.
Oh yeah one more thing
(7) Be a rat bastard DM and don't cut'em any slack.