One of the best piece software for Android is Torque.
Torque is an ODB2 diagnostic app than can be used to clear error codes and diagnose your car. Pretty much all cars after 2003 or so have ODB-2 as a standard protocol. You can buy a ODB2 reader at your local Kragen or get your fault codes cleared at your dealership. Torque, brings that capabilities to your smartphone.
This is a $5 app that deserves some attention and praise. To use this, you need a bluetooth ODB-2 reader. The most common is the ELM327 bluetooth dongle that can be found on ebay. Most of them are cheap no-name Chinese no name ones. I wouldn't be able to tell you which one works better than the other. I got lucky, mine works.
I use it just to fart around. Nothing serious. You can time your zero-to-sixty times or measure the oil pressure if your car doesn't have a oil gauge. Frugal types can check your MPG in real time. There are countless of things you can do with and it helps you get a better understanding of your car. All my cars are fairly new so I don't have to worry about clearing fault codes yet. There is a GPS tacker built in so you download your tracks. You can use this as a blackbox computer recorder. .
I do have one funny story to tell. Last year, I was looking to buy a used truck and brought my ODB2 setup along. The seller (in a used car lot) panicked when I told him I was going to check the car for any error codes. He quickly shun me away and didn't want to sell me a truck any more.