The Adobe Beginner Classes Episode #13 (project file) and Episode #14 from Dennis Radeke show you how to extrude text into 3D in After Effects. Dennis credits Danny Princz and FXPHD.com for this refinement of the old duped layers on Z-axis technique.
Previous posts Advanced Extrusion with Shatter and Maltaannon extrudes AE Shapes discussed other tutorials that extrude layers into 3D without 3rd party filters like Trapcode Echospace, Boris Continuum or Zaxwerks Invigorator. Yet another option is Photoshop 3D text extruded with bevels beyond Illustrator, like Zax, with Swift 3D PS.Here's the Adobe tutorials (full size at AdobeTV; #13 starts at 5:00 & #14):
Update: Harry Frank posted a free graymachine 3D preset to create 3D extruded text in his video tutorial Retro Style Graphics.