AVR-Chat

> All I need is a USB based little blob with an MCU inside to 
> get the data from sensor or via ADC and send it to the PC. A 
> little program on the PC would display the data. There are 
> AVR devices that have pretty good spec like built in USB and 
> ADC. (like ATmega16U4, AT90USB162 etc)

You might want to consider the Arduino platform/environment and in
particular this module 
http://www.arduino.cc/en/Main/ArduinoBoardNano which has a bunch of general
purpose Digital and Analog I/O and I2C and SPI etc to interface to common
sensors. 

Also there is some firmware called Firmata:

http://www.arduino.cc/playground/ComponentLib/Firmata

which is essentially turns the Arduino hardware board into a remote
controlled I/O device that can be controlled from various high level
languages on the PC.

HTH

Alex