For all the mechanical engineers...

[Psycholist]

If you're not afraid of technical depth this is a very good source of information on bikes, particularly relating to wheels: http://www.wheelfanatyk.com/

While I'm posting here www.sheldonbrown.com should also get a shout out as the encyclopedia of obscure bike part sizes. Thanks to all the axle standards, wheel sizes/widths and boost, this may be much more important in future as for most of the 90s and 2000s mountain bike part sizing was very easy...