www.expertflyer.com is what you want (or the KVS tool from our resident FTer, err, KVS)...

Note you can view the currently allocated seats this way but it won't really tell you how full the flight is - if it's a 744 with only 1 Gold in F in 1A and no other status pax or full fare pax in any cabin it could be nearly rammed full of discount ticket holders and you'd never know from the seatmap alone! Extreme example with a few logical flaws (at some point, they'd only be selling full fare tickets I'm sure) but you get the idea