We've created, appointed, or otherwise identified certain question/answer pairs that are the canonical answer for certain problems. These are answers where the community has said all it is going to say on a class of subject, because although everyone's particular problems in that class are somewhat different, to the extent that they are interesting they aren't different, and to the extent that they're different they aren't interesting (to anyone save the questioner). So we write one answer that's designed to be the last word on the subject, and say no more.
A list of these problems is given in the (soon-to-be) accepted answer below.
Post an answer (to this question) to nominate a question for inclusion. Questions explicitly posted for the meta-approved purpose of creating a canonical Question/Answer can skip this. Consensus seems to have evolved that a score of +4 is enough to approve canonicalisation.
Delete answers nominating questions that have been merged into the official list. Try to keep this post neat and tidy.