I am a big fan of simple Web tools that make annoying tasks easier. One of the latest tools in that vein: WhosOff, a free Web service that helps you coordinate vacation and out-of-the-office times for people in your company.
There are plenty of online in-out boards available (many are downloadable apps--feh). What makes this one somewhat different is it has smarts. You can set up rules to make sure your two customer service people aren't out of the office at the same time. The system also tracks vacation time that accrues annually, and won't let staffers book more time off than they have coming to them. You can set WhosOff so employees can't lock in vacation time without getting their manager's approval, or so it books time automatically (providing the time off isn't breaking any conflict restrictions you've set up). WhosOff e-mails requests to managers and confirmations back to employees, and it looks like once the system is set up and running, it'll be easy for everyone to use.
When you begin to dive in to WhosOff you realize scheduling vacation and out-of-office times is a lot more complicated than it appears at first blush. For the system to run well, it needs to know who reports to whom, how much time people accrue, and which staffers can't take off at the same time. For those reasons, setting up WhosOff is a bit more complex than I'd like to see, but for the most part the interface is clear. WhosOff does not appear to integrate with payroll or personnel systems, though, which means you'll have to maintain your company directory separately on WhosOff.com, and make sure you transfer vacation and sick time into your payroll system after the fact.
You can spend a lot of money for a full-on human resources program that has this feature in it, but for a smallish office that doesn't yet have the resources, WhosOff is a good way to help people coordinate their vacations and off-sites.
Related scheduling tip: To create a simple self-service tool customers can use to schedule appointments with you, see AcuityScheduling. It's just too bad Acuity and WhosOff don't talk to each other. Perhaps someone could mash the two together?