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[Mysterious Stranger]

*shrug*

You're not going to find an abandonware RPG as good as Fallout. Similar in some regards maybe, but that's about it. I suppose you could try the Magic Candle series and Bloodstone, they're turn based and non-linear... Betrayal at Krondor is excelent, IMO, and freeware. You could also try some of the old D&D/AD&D games... maybe the Dark Sun games. I think they're abandonware...

If you're willing to spend money your options will increase, but they still won't be like Fallout.