I stay connected with most of my professional network on LinkedIn, it’s particularly popular in the technology sector. A newer tool that I’m finding really useful is Highrise, its a simplified approach to Customer Relationship Systems and enables me to keep personal records of all the connections I make. Recently I’ve found that Facebook is fast becoming the single place where almost everyone I know is staying connected - not just people I know from University.
Obviously none of these things are any substitute for a face to face meeting or phone call.