Firefox: If Gmail's web site is your one-stop shop for organisation, previously mentioned ScheduleOnce's Firefox extension is a great meeting plug-in. The service allows you to plug in when you're available, invite others, and figure out the best meeting time.
ScheduleOnce formerly relied on Google Calendar to work its best-time-finding magic, but its newest add-on, Meeting Scheduler for Gmail, plugs right into Gmail's sidebar. Click "New Meeting" and you'll get a pop-up asking you to set a meeting date, or range of dates, and the times you're most available. Plug in other email addresses, and ScheduleOnce goes out and grabs their own best times. All of your meetings are tracked in the Gmail sidebar, and can be optionally loaded into Google Calendar as well.
Here's a clip of ScheduleOnce for Gmail in action:
ScheduleOnce's add-on is free, works wherever Firefox does. No sign-up required, but a Gmail and (optionally) Google Calendar account is required.