Here are a few things you can do with your new-found number-entry tool:
Search, Add and Edit entries on Barcodepedia (A wiki-style barcode catalog)
Set up an archive of your DVDs CDs, Books, Comics, Games, etc. Try a quick web search.
If you really wanted, you could print out certain commands or long strings you type often (like http://erroraccessdenied.com for example), print them off in Code 128 format, and scan them as required instead of typing. This would be an interesting way to store passwords.. Folks would have no idea what a cue-card with barcodes on it is for.
If you can figure out anything else fun to do, please feel free to post about it in the comments section for this article.
Ok, here I have a Symbol LT-1780 handheld barcode scanner. It has an RJ-45 cable with a 9-pin end (Not RS-232). I took an old RS-232 plug and put that on the end of the cable, and used that to wire it up to the Arduino.