iOS: Good Habits helps you keep track of multiple activities and habits that you want to build over time. The app shows you a calendar of your current and longest unbroken chain lengths, shows you your tasks as a checklist, and helps you stay motivated to keep your habit chains unbroken.
We've discussed how helpful Seinfeld's productivity secret (aka "Don't break the chain") can be, but to give it a try you'll need a good calendar or tool to tack your habit chain. Good Habits is a free utility that fits the bill nicely and includes some additional features that offer visual cues to keep you motivated so you don't break the chain. Overdue items are marked in red, and if you're sick, travelling or can't hit your to-do for some reason, you can always pause it so you can return to it without having to start a new chain. The app is open source and available now in the iTunes App Store.
Good Habits (free) [iTunes App Store]