AVR-CDC is a USB-RS232C interface using the CDC (Communication Device Class) protocol. Although the bulk endpoint required for CDC is not allowed within the low-speed USB standard, it works fine on major platforms.This ingenious project implements the USB Communication Device Class, making use of the operating system’s driver for the device class. No separate driver is needed on the host! Although it goes somewhat beyond the USB specification, it works on all major operating systems.source

USBasp is a USB in-circuit programmer for Atmel AVR controllers. It simply consists of an ATMega48 or an ATMega8 and a couple of passive components. The programmer uses a firmware-only USB driver, no special USB controller is needed.
Features

• Works under multiple platforms. Linux, Mac OS X and Windows are tested.
• No special controllers or smd components are needed.
• Programming speed is up to 5kBytes/sec.
• SCK option to support targets with low clock speed (< 1,5MHz).
• Planned: serial interface to target (e.g. for debugging).

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AVRUSBBoot is a bootloader for the Atmel AVR controllers. It uses a firmware-only USB driver to transfer binary data from the PC to the flash of the controller. Once the AVR is flashed with the bootloader, no other ISP programmer is needed; then the microcontroller can be reprogrammed over USB.
Features

• Software works under multiple platforms. Linux, Mac OS X and Windows are tested.
• Tested with ATMega8-16
• No special USB controllers or smd components are needed.
• Bootloader fits into 1024 word bootloader block

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USB power bus is a great source for charging a single-cell Lithium-Ion battery. This circuit shown you how to build a USB-powered single-cell Li-Ion battery charger using National Semiconductor s LM3622 Li-Ion Battery Charger Controller.The battery-charger circuit is designed to operate as a high power USB function. To be compliant with USB Specifications (Rev. 1.1), a high-power function must not draw more than 500 mA from the bus during normal operation. The LM3622 uses the 0.25 current-limit resistor R1 to set a 400 mA maximum charging current. This leaves a 100 mA surplus that can be used to supply USB control circuitry and other functions in the device.source

This small realization, based on one of the most common IC (MAX232) is designed to create a small and convenient TTL to RS232 and vice versa convertor.All you need is 4 caps, one IC and 2 connectors. If you want to add a small regulator on the board (already foreseen on the PCB) you just need to add a 78L05 regulator and a cap.If you do not need to regulate the power supply, you can just replace the 78L05 by a strap (pin1 to pin 3) and drop the 78L05, C1 and C2.source