During our vacation, I encountered an error downloading photos off my CompactFlash card (very bad.) Despite my prodigious troubleshooting efforts, I was unable to get the photos off the card. As I sat in frustration that morning in the hotel room, I was sure these were undoubtedly the best photos I'd ever taken.
I then remembered a conversation I had with a developer at Photokina (photography trade show) in Cologne, Germany a few years ago. He made a product called Photo Rescue for just such emergencies.
A quick download and some munging around later, I got all my photos off the card. They turned out not to be the best I'd ever taken, but they were important to me.
I highly recommend this app. I was well worth the $29.