Addons are perfect for browsing gear in game, but if you want to check out some gear without launching the game client, there are alot of useful websites as well.
- MogIt - An AMAZING addon that I just recently downloaded. You can preview an extensive range of items and put together sets with this addon, and alot of the screenshots I upload to my blog are previewed with this addon.
- AtlasLoot - The addon I've primarily been using while creating my transmog sets. It is a very useful addon, but a bit flawed when it comes to creating outfits since it doesn't include alot of greens or blues.
- World of Wardrobes - A great website where you can browse gear in different categories, based on armor type or colour.
- WoW Roleplay Gear - Lists alot of sets available for the different armor types.
- MogMyGear - A very clearly structured website where you can preview different green, blue, epic and heirloom sets depending on which class or race you pick.
- Icy Veins - Allows you to browse items, categorized either by armor type or item model.
- Disenchanting Azeroth - A blog full of reviews on weapons and weapon designs.
- WoW Tabards - Not everyone uses tabards, but they can do wonders for some transmogs. This site shows all the tabards available in game and where you get them.