How can I keep everyone in the office on the same page?

I work for a small business and have been trying (unsuccessfully) to provide a solution for our outdated calendar. What I am looking for is an instrument that will allow all of our management and technicians, on and off-site, to have the same, most current calendar on-demand. This includes syncing with our desktops at the office, laptops on the road, and multiple mobile devices (blackberry, android, iphone) for every user. I am open to all suggestions, but my budget (financially and temporally) will be limited.