A USB port is a great power source for charging a single cell li-on battery. It is capable of supplying maximum 5.25V and 500 mA. This circuit is a USB powered single cell li-on battery charger. LM3622 is used as the controller. This special purpose IC has a precise end-of-charge control and low battery leakage current about 200nA.source

A SEPIC (single ended primary inductor converter) is a DC-DC converter which allows the output voltage to be greater than, less than, or equal to the input voltage. The output voltage of the SEPIC is controlled by the duty cycle of the control transistor. Here is a USB to 5V SEPIC DC/DC Converter with Short-Circuit Protection.This circuit is ideal for applications where the output must reduce to zero during shutdown. The input current limit not only helps soft-start the output but also provides short-circuit protection, ensuring USB device compliance even under output fault conditions.source

This is a USB to 12V Boost Converter circuit.It use the IC LT1618 from Linear technology. The converter has a selectable 100mA/500mA input current limit, allowing the device to be easily switched between the USB low and high power modes.The efficiency can exceed 85%. If the load demands more current than the converter can provide.with the input current limited to 100mA (or 500mA), the output voltage will simply decrease and the LT1618 will operate in constant-current mode.source