Android: It's not easy to gauge your blood alcohol content without a breathalyser. Blood Alcohol Finder for Android helps you get in the ballpark by estimating your BAC levels based on your size, gender and amount you've been drinking over time.
The app requires a certain amount of self-tracking, so it's not exactly going to be ideal to use while you're out on the town. As a tool for self-analysis, however, it's useful. Not only will it show you what your BAC level is estimated to be at now, but how it might change over time. Just in case you were wondering how long those four rum and cokes are going to be in your system.
As with any tool like this, it's important to note that this is not a substitute for safe drinking habits. We all have different tolerances and reactions to alcohol and understanding our own personal limits is essential to staying safe while imbibing. This app may be useful for putting numbers alongside those gut feelings, but keep in mind that a low number on your phone's screen never means you're good to drive or that your judgment is sound.