Some JSON parsers do offer incremental ("streaming") parser; for Java, at least following parsers from json.org page offer such an interface:
(in addition to Software Monkey's parser referred to by another answer)
Actually, it is kind of odd that so many JSON parsers do NOT offer this simple low-level interface -- after all, they already need to implement low-level parsing, so why not expose it.
EDIT (June 2011): Gson too has its own streaming API (with gson 1.6)
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