Marc Andreessen and Eric Bina with Mosaic / Netscape

Larry Page & Sergey Brin with Google

Dan Bricklin and Bob Frankston, VisiCalc

Pierre Omidyar, eBay (some may say it's not an impressive technical feat, I disagree)

Bill Joy did plenty of amazing work before 30 (including co-founding Sun)

Sandy Lerner founded Cisco and did some amazing technical work pre 30

Bob Metcalfe was around 27 when he created ethernet