Phil Plait has thoroughly debunked Richard Hoagland's claims of civilisations on Mars on his excellent site Bad Astronomy. My favourite has to be the tower blocks on Mars, which are really just JPEG artefacts blown up a thousand times.
Phil also addresses the famous Face and links to a high resolution image of it taken by Mars Global Surveyor. If you squint your eyes while looking at this image, the face becomes more apparent, illustrating the fact that it was caused more by the poor resolution of the Viking image rather than anything else.
While Plait's article is, like the rest of his work, well worth reading, I do wonder why he's gone to so much effort to debunk Hoagland's claims. Looking at Hoagland's site from a web design point of view, the multi-colored text on a black ‘star field’ background, the new-age graphics and the general sensationalist tone do tend signify the work of a conspiracy theorist, new age cultist type whose claims almost always have no scientific basis whatsoever. That said, I have found one site that seems to buck this trend: The Campaign for Philosophical Freedom is well designed (with CSS!) with nicely presented articles on a variety of subjects. Delve deeper and things start to get a bit weird as you discover that its founder has “scientific proof” for life after death and the key aim of the site is to deliver his case for this. Right.