Electronic: Home School Tracker. They also now have an online version but I really see no need in switching. The 1x fee paid down-loadable version includes updates. I used the free version for a few years then bought the paid version about 4 yrs ago. One good thing about the paid version is that there are over a dozen yahoo groups set up to exchange lesson plans. The free version does not have the lesson plan feature.
I have a paper one in my purse for the "I have to go to those things", things. I have one online, with everything on it. I have another one that I print out and put on the side of the fridge and check all day, mostly for household chores. I have a desk calendar I put bills on.