AVR-Chat

As long as you aren't going to need more than 127 controlled by the same
master I2C sounds just about perfect for the application.  You specified:
2 - outputs
1 - input
1 - interrupt
1 - ADC
Serial Communication ( 2 pins for I2C )

With the above requirements I'd say that the 2313 is about perfect at the
moment.  If you could wait for the new ATTiny24/44/84 series of 14 pin chips
you can save yourself some space (14 vs. 20 pin, not bad).

Either way you have built in hardware support for TWI (I2C) serial, a proper
external interrupt, and the ADC's you require.  The extra pins, 4 on the
24/44/84 and 10 on the 2313 can be used as hardware address selection lines
that would allow 16 and 127 ( actually 1024 with all 10 pins but I2C doesn't
support that size address space ) possible addresses respectively.

If timing is vitally important you'll want to run an external crystal and
that'll take two of your pins, in that case I'd recommend the 2313 since the
24/44/84 would only allow 4 addresses at that point.  If all you needed was
4 slave controllers there's no problem but...

Martin Jay McKee