I've looked in to a number of solutions, none of which were really great. There a bunch of online calendar apps that everyone can share part in such as google calendar and 30boxes. We've been using webex office weboffice.com and it does a decent job. We use it as a whole online colaboration site to post schedules, job related issues, client documentation, web links, PO requests. They even have modules for expense reports and time sheets.
Microsoft just released one called Office Live I think. I was among the beta testers and I absolutely hated it - but it may work better for you if you're using outlook.