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While I have not used these myself, as I have moved past BASIC to scripted languages, I did some quick Googling and came across two possible incarnations of OLD BASIC, not a newly created one.
One place to begin searching is the Wikipedia page on BASIC.
For Unix, there is Bas, a BASIC interpreter. It alleges to be compatible with the BASICs of the 1980s. I have not tried to use this myself.
In order to use it, you would likely need a Linux/Unix or Mac machine. You would also have to compile it. (It is written in ANSI C.)
It should, theoretically, compile on those three systems (at first blush).
There is also Chipmunk BASIC for the Mac. It is allegedly like the older BASICs as well.
Bas -- BASIC Interpreter
http://www.moria.de/~michael/bas/
Chipmunk BASIC
http://www.nicholson.com/rhn/basic/
Hope this helps. I have many fond memories of GOTOs and Returns myself.
Sincerely,
John Nelson