There's 1001 places on the web where bicycles reviews can be found, unfortunately I don't know of a site which has consistent trustworthy reviews, and which hasn't been overrun with immature idiots.
Bike and
Dirt Rag do a pretty decent job of reviewing things in print on the mountain bike side, but of course they don't come anywhere close to covering the entire market.
Best bet as
bikes and beats noted is to identify your needs, ask yourself what you like & don't like about your old bike, what you want to improve on, what you want to do with your new bike and a rough ballpark figure on cost. Then find a good bikeshop, one where they guys actually
ride their bikes instead of driving to work in SUVs, an almost sure sign of goodness is a store which organizes & leads regular group rides. Talk to both the sales guys and the mechanics, some bikes are technological marvels and ride really well but are prone to frequent & expensive breakdowns, the sales guys might not know about it but the mechanics sure will.
And make sure you test ride the bike. Always test ride the bike.