We have just added a feature to our short URL redirection project GentleURL that allows the users to create their own identifier instead of the machine generated code.
http://gentleurl.net/gentlesource/
Only numbers and alphabetic characters are allowed. The identifier is case insensitive. That means that if "gentlesource" is already taken, no "GentleSource" or "GENTELESOURCE" can be created.
BTW: In case someone is looking for a short URL redirection script, he does not need to build one from scratch (http://gentleurl.net/ONXAB/). Just contact us. I am sure, there is something we can do.