There's nothing wrong with mirroring a site for one's personal use. In fact, unless one turns off caching, it's impossible to browse the web without making a temporary copy of whatever one has been browsing. If someone puts something to make it publicly available, it means just that, that it's public ally available. This doesn't mean one has a right to post the material but one does have a right to have the material (if the author isn't O.K. with that, the author should not have made it available in the first place).
TreasonousBastard wrote:
Assuming you do, I would also assume you have the site owner's agreement. You don't just go ahead willy-nilly mirroring sites.
While I guess it would always be polite to ask for permission before spidering someone's website (particularly if one has a fast connection which could put an excessive load on their server --not an issue with me but it could be an issue with others), this isn't always possible (such as if you're mirroring a dead person's website --yeah, I know, I need a new hobby :P).
And, of course, one shouldn't discount the fact that there's a tradition of activism by mirroring in instances where the "powers that be" (or whoever) conspire to keep information from being available from the general public. This is why, for instance, one can find a lot of information about the "secret" writings of the Church of Scientology (they have a tendency to attack those who expose them for what they are through the, often successful, misuse of copyright law). Often, one could say this falls more under the realm of civil disobedience (so yes, generally, one shouldn't "go ahead willy-nilly mirroring sites" but sometimes somebody has to).