This is one of the smallest if not the smallest arduino.
It has 6 Digital I/Os
All pins can be used as Digital I/O
Pin 0 → I2C SDA, PWM (LED on Model B)
Pin 1 → PWM (LED on Model A)
Pin 2 → I2C SCK, Analog
Pin 3 → Analog In (also used for USB+ when USB is in use)
Pin 4 → PWM, Analog (also used for USB- when USB is in use)
Pin 5 → Analog In
I had some problems to make my PC associate with the USB driver. In fact the ArduinoISP was still unrecognized, but the Arduino IDE still uploaded the sketch. I had to download different drivers. Below I will post a link of some useful sites.
Another thing to say is that this does not work like an arduino when connected to a USB, even the upload is different.
Having said this, one can use the arduino IDE to upload the software.
One needs to compile to check the program for errors. Then hit the UPLOAD button with the ATTINY85 DISCONNECTED. In the bottom window, it will tell you when to plug the ATTINY85. Then it should upload.
Use the Example of the Blink, but instead use PIN 1, as this is where the on board LED is connected to.
My Arduino IDE is 1.6.5 on Windows XP.
Some helpful links
Got one of the drivers from here > https://github.com/adafruit/Adafruit_Arduino_Board...
More info from the supplier
Links were active on 1st October 2015.