In an age that worships fame and wealth it's inevitable, I suppose, for most people to remember the celebrities who took classes at ImprovOlympic: Chris Farley, Andy Dick, Mike Myers. But what makes that talent factory remarkable--and partly accounts for its longevity--is how egalitarian it is, churning out scads of strong, funny, committed performers. Most ImprovOlympic alums are not famous. Or they're famous only among other professionals: Brian McCann, who writes for Conan O'Brien (and occasionally performs on his show), is respected by his fellow comedy writers, and teacher-director Susan Messing casts a long shadow in Chicago's improv world and almost no shadow outside it. But in the long run, these artists have had more influence on American comedy than any out-of-control celebrity. Moreover, some of the best improv I've seen has been created by people too raw, spontaneous, and confrontational to get hired by SNL. For ImprovOlympic's two-day 20th-anniversary fest, a number of teams will reunite--among them Blue Velveeta (McCann's and Messing's team) and the Family (many of whom were founding members of the Upright Citizens Brigade)--and other alums will perform. ImprovOlympic founder Charna Halpern promises that Del Close will make a posthumous appearance. Athenaeum Theatre, 2936 N. Southport, 312-902-1500. August 3 and 4: Friday-Saturday, 8 PM. $25.