After 18 seasons and 400 episodes, Fox's The Simpsons (Sundays at 8 pm/ET) has attracted more than 350 celebrities to offer their voices to animated doppelgangers. Some have played themselves (Steve Buscemi, anyone?), some new characters (Reese Witherspoon as Rainier Wolfcastle's daughter, for instance), some old characters (Kiefer Sutherland and Mary Lynn Rajskub, who reprise their roles from 24 on May 20) — and in one instance, a mix (Elizabeth Taylor played herself and voiced Maggie's first word). Throughout, producers have attractread more