The HiCoffee app for iOS has been a great resource for me. It allows parameterized inputs for your personalized caffeine half-life, bedtime, bedtime caffeine levels, and daily caffeine limit. As you log your intake, it shows a nice chart of caffeine in your body throughout the day. The visualization has really helped me understand how caffeine is affecting me, and it has been very helpful in curbing my caffeine intake when it has gotten away from me. As an added bonus, the app has a lot of caffeine data on common drinks on the market.