Manage and Track Maintenance Schedules
There are many different philosophies about how much maintenance a bicycle needs, and on what schedule or basis it should be performed. 

As I prefer to never be worried about whether my bikes will fail under me on a ride, I am more meticulous than most.  I currently have 4 bikes, each with different function.

I service each bike when it's time to change the chain. I do that at 1000 miles ridden. I have yet to need more than chains, cables, and lubrication, and I've never had a more serous mechanical than a flat tire on a ride since I started this maintenance scheme. 

Your question, though, is about how I keep track of that regime.  I've tried several things, including an Excel sheet, a Rails based web app I built, and just logging the miles on paper, but the problem with all of them is that they are dependent on me to remember to use them. 

Given my chosen schedule, if I forget to log a day, or a week, of riding it's not going to hurt me. 

But what I'd love to see is an iPhone/Android bike computer app with the ability to set more than 2 bikes, and to set maintenance reminders based on GPS miles logged for each bike. Perhaps with an "x distance or y time, whichever comes first" based reminder, with the mileage and timeframe user adjustable. 

Haven't found it yet though. My bike computer will let me set a mileage reminder, but I'd need one on each bike.