AVR-Chat

For output lines from the 5V to 3.3V system, a simple resistive divider will do.
For inputs, check the data sheets, but in almost all cases you'll find
that the guaranteed levels on the 3.3V system are high enough and low
enough to be valid 1's and 0's to the 5V system without modification.
Bi-directional lines are more interesting, but still not that hard.

Maxim loves to charge you $5 to solve a $0.05 problem.