If you need to power a 5V circuit with a 3.7V battery you can use this simple Sparkfun charger. It can provide up to 1A of current to its output
The circuit is built by feeding a charge controller IC MCP73831 to the LiPo port and the input of a boost controller PAM2401. Various connection types are provided for battery, charging source, and switch to allow application flexibility, and two LEDs provide information on system status. If you need more than 5V from your project, SparkFun LiPo amplifiers / chargers can also be daisy-chained for a higher return on your investment.