Over the years, I've tried a lot of organizers and planners, but no system is as convenient and easily customizable as my trusty DayRunner. The simple black leather planner is sleek and professional, with a zipper closure to contain all the papers, business cards, receipts and other paraphernalia that I tend to deposit there. The convenient 3-ring, 5 1/2" x 8 1/2" format easily holds month at a glance or week at a glance calendars (or both, if you choose!), as well as multiple other planning tools. You buy only what suits your planning style. There are options for Finance pages, Telephone/Address pages, Project Planning pages, storage pockets, and the list goes on and on. The multiple calendar options are great for scheduling short- and long-term activities, and I like that the refills and add-ons are available at all office supply stores, and are all quite affordable.
I've relied on my DayRunner in my professional and personal life for close to 15 years, and would be absolutely lost without it.