CompanionLink for Google Calendar™ is a two-way synchronization solution for people that want to extend their Google Calendar onto their desktop calendar systems or mobile devices. It can synchronize Google Calendar with all the latest Pocket PC, Windows Mobile, Palm, and BlackBerry devices in addition to Outlook, Lotus Notes, Palm Desktop, and Groupwise applications. Not free unfortunately.
EDIT (27/06/06): Wonderful, the latest demo crashes whilst reading my Outlook 2002 calendar.
EDIT (Feb 2008): Seen this entry gets a lot of hits – I now use Plaxo to sync Outlook & Google Calendar – it works very well and is free. Be careful on some of it’s more “social networking” elements however.