The serial UART is a common interface used by many chips and devices. Ginpb designed a USB-to-UART adapter that works with 3.3V and 5V UART signals. The device is based around the FTDI FT232BL USB-to-UART IC with an added 3.3V power supply. The different interface levels are selected by an on-board jumper.
Simple USB to UART adapter that works with 3.3V and 5V UART signals - [Link]
This project shows how to build a STK500 AVR ISP programmer with USB support. It uses FT232BL chip to implement USB to serial converter and ATmega8 in DIP package. This programmer is a clone of original AVR STK500 programmer, it works with AVRStudio and AVRDude and all main platforms including Windows, Linux, BSD and MacOS X. Check schematics and PCB on the link below.
STK500 compatible AVRUSB programmer - [Link]