XP preaches “traveling light,” preferring conversations, working code, and tests over voluminous documentation and specifications. A few physical artifacts are useful, though. In particular, the planning game (see Business Practice 2: Play the Planning Game in Part II) uses index cards heavily. The name gives it away; these are the game pieces by which you arrange your schedule.
Every bit of information in XP can be tracked electronically, through email, project tracking software, a spreadsheet, or a web page. In some cases, electronic tracking may be the best solution. Regular index cards work really well, though. You can write on them, stack them, tear them in pieces, and tack them to the wall. They’re satisfyingly tangible, easy to carry, and require no special tools other than a pen.