I need to read different bar codes, and would be great that I can use my iPhone for that. Currently I have an application to read QRCodes but I'm looking to read PDF417 too.
RedLaser for "normal" codes and ZBar for QRCodes.
These are the fastest and most stable I could find. Plus, they are both free.
Red Laser has just updated to read more types of barcode. Plus it's free.