Is it reasonable to register an .org variant of a famous .com domain name?
This is probably a good way to get some traffic together with some legal problems. If the .com name that you aim at is a trademark, this could definitely get you in trouble. If the .com name is a word or a combination of word from a dictionary, you are less likely to have problems. Mind that some dictionary words like apple and windows can be trademarks, as well. Furthermore, many companies buy the corresponding .net and .org domains when registering a .com domain. Anyway, the choice is up to you.