USB flash recovery tools?

I've got a 512MB USB flash keychain. Tonight I tried to use it and it didn't work. Ack! So, after some cleaning of contacts and messing around I got it to be recognized on my Mac mini, but with no filesystem. I dd'd the contents to a .iso file so I'd at least have some sort of backup. As it stands, the backup file is smaller than it should be (480MB?) so I'm not too confident in it. Anyway, can anyoe suggest a recovery tool for the drive itself, or to process a block dump? Either Mac OS or Linux tools would be good. I'm fairly sure it was in FAT32. So far Googling tends to bring up demos and trial ware.