What iPhone maps app can I use if I don't have a SIM card?
Navigon
TomTom
CoPilot*


Navigon offers different apps for worldwide coverage.

Depending on where you are headed TomTom has several different apps for particular regions. Scroll down on the iTunes page to see some options.

*It appears that CoPilot offers directions when offline, but not turn-by-turn directions. 

On all of these apps be sure to check and verify that they provide maps for the country (or countries) that you will be visiting.