MontySylver:
I used an ATTiny85 to send serial data and an Arduino Uno to receive it. First I got some strange gibberish too until I pulled the ATTiny's VCC out by accident. Without VCC on the ATTiny it sent what I wanted, after reconnecting VCC it was gibberish again. There were no pins except Tx, Rx and GND connected.
That's BAD. It means the chip was drawing power from the TX line (or something).
The 8mHz internal clock varies with temperature and voltage. I'm guessing you got lucky and the bad power source made it work but, DON'T DO IT AGAIN. Ever. You can damage the chips.