I use Active@ File Recovery (and sometimes Active@ Partition Recovery for really awful instances). It's relatively inexpensive, and you get free upgrades for awhile. It works on hard drives, flash memory -- pretty much everything, and does a spectacular job at it. I've recovered nearly obliterated hard drive partitions using partition recovery and then used the File Recovery portion to rescue the files.
http://www.file-recovery.net/download.htm
There is a free demo as well.
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If you want something totally free... you can try this:
http://www.piriform.com/recuva -- it's rated pretty highly by CNet.

(CNet link:
http://download.cnet.com/Recuva/3000-22 ... 53287.html)
**Edit: I should also note that if you continued to use your flash drive after accidentally deleting the file, it is entirely possible that no recovery software will be able to retrieve it. The more you use the drive, the more likely the memory where the file was originally stored will be overwritten, and the original file will be lost forever.