There are few things better than reconnecting with old friends. Except when they're now-famous old friends on TV. We're heading into a surefire winner of a doubleheader this spring: Megastar George Clooney's Dr. Ross
will check back into "ER" tonight as the long-running hospital drama prepares for the big goodbye on April 2, and Shannen Doherty's bad girl-gone-good Brenda will resurface
-- with child in tow? -- on "90210" for its season ender. But not all reunions have resulted in heartwarming nostalgia. We rate history's biggest TV comebacks, from David Duchovny's triumphant return to "The X-Files" to Clay Aiken's awkward fan encounter on "American Idol."