TLDP maintains really good documentation ...at least from a technical content perspective: their presentation is mostly rubbish (like 1990s HTML).
Beyond their, StackOverflow, the man pages and the O'Reilly reference books (thank god they're electronic, now) are all solid. StackOverflow has the benefit of being more "bite-sized" and the answers you find tend to align very closely to your specific need.
But, what really helps is knowing exactly what it is you're trying to achieve, the technical terms for it and then how to make Google dance with those terms. =)
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