I use google calendar. It ties with my email and also updates on my phone, so I can change it whenever I need to. Beats carrying a daytimer around too.
I only use it for important things like calls with clients, appointments, etc. but I've found when I start scheduling in workouts and other day to day stuff it definitely improves my productivity. Another bonus is that you don't have to juggle all these thing in your head. Just set it and forget it, brah.