Branko Jovanovic’s answer cites Yahoo Answers and Reddit as the more informal alternatives; the more formal alternative is Stack Exchange. The Stack Exchange family of sites started with Stack Overflow, where it has the monopoly as a programming advice site (that Quora could never hope to displace), and is aggressively moving to other fields; its English Language site is well established in the googles too.
FWIW, Nikolay Starostin’s answer to Is Quora overtaking Stack Exchange? indicates that Quora has been ahead of Stack Exchange this past year, but not by an overwhelming margin; and Quora’s numbers are skewed by its high Indian population.
Outside of English, Zhihu may be a Chinese knockoff of Quora, but it guarantees that Quora won’t even bother to step into the Chinese space; Mainland Chinese are well known for not venturing outside the Great Firewall. TheQuestion may ending doing the same thing with Runet. OTOH, it does not seem that the French and German existing alternatives to Quora pose a serious challenge.