I just use Google calendar:P When I have something done I put it on the calendar, then calculate forward to when I need said-thing done AGAIN and make a note of it (i.e. something like "Deworming: 7 Week Mark", so I know what's coming). You can also set it up so Google will email you when an event comes up on your calendar. Also use it to record weight checks, out-of-the-ordinary vet visits, weird weather, horse shows, etc.
Since my job has me sitting in front of a computer 8 hours a day, this works for me. I also back it up in a small event planner for travel, or if I need to make a quick note while I'm at the barn.
Pretty easy, but I only have one horse to worry about.