You could use Wowhead's character viewer. You can swap gear on there and it'll display.

Use this site. It's pretty awesome. Figured out what Hunter gear I want through this. :3

http://www.wowhead.com/transmog-sets

Use the Mogit addon. It is amazing.

I also found a fun site called The Visual Roleplay Gear List, if you don't want to go the addon route.

i'll second the mogit suggestion. it's modular and load on-demand so pretty lightweight(if that's a concern).

the best part is making a wishlist, and if you use atlasloot, you're totally set as it links to atlas to provide a drop location/vendor list for your wishlist. it also has full functionality with the dressing room option to give you full preview capability.

a half hour's work unless you're really fussy will let you build up a wishlist of all the gear you'll need. i used it to get a ninja suit for my rogue, as well as decide that the season 8 pvp gear prolly is prolly the best worgen DK set to use for its low profile and very sleek design.

*chugs a pepsi while ford mustangs leap through exploding hoops in front of a giant screen showing an x-box*

yeah, that was nigh-endorsing an addon.

it's also the only non-combat addon i have. that says something for how bloody nice it is.

This guy has excellent resources listed:
http://dreamsofisorath.wordpress.com/2011/11/23/transmogrification-resources/

http://wowmogging.com/ has literally every set, for every class and proficiency, it also has collections (Sets that look alike and match but don't count as a set e.g. hillman's leather)