I thought I'd make a note here about a great bit of software we wouldn't like to be without. It's called share360 and it's a server based diary that everyone in the office can use. Actually it's a bit more than a diary, it's a "groupware" program which can handle email, form processing (like holiday forms, or any other business form you have that one person writes and another has to approve), project management, and several other things that help people work together in an office. We only use it as a diary system though.
The clever thing is that it runs on a web server (read web server as any old computer set up to server web pages - an old Pentium 2 within the office network would be fine) so that there is no software to install on each PC. The interface allows any user to add/amend/edit any other users diary (assuming they have the correct permissions). It's one of the few programs I could find that allowed such simple group diary editing (Outlook only works for a single user unless you get expensive things like MS Exchange Server). We couldn't live without this as any one of us in the office can book an appointment for anyone else.
We've had it for a couple of years and it has worked flawlessly in all that time. Better still, the makers are improving it all the time and minor feature updates and improvements are free. We paid a one-off licence fee some years ago for up to 10 active users, but it can handle hundreds on suitable hardware.