Using Clipse to schedule meetings with my clients (outside my company) is a game changer. Clipse lets any us see when we're all available, even for a group of people. If they're not already using Clipse, the meeting link works great and I like how it allows them to choose the subject and even upload a file to the meeting invitation. Calendar add ons aren't the most common for me, but once I added it to Outlook it's incredibly convenient.