Wowequip is a simple tool for trying on gear. If you are tired of doing math for yourself to see what stats you will gain/lose by swapping in a new item this is a great program. Wowequip allows you to accurately model all of your in game stats, items, enchants, and even talents. Wowequip is a free windows based program that you can get directly from their website.
A guild hosted online tool, the Character Planner exists too. It is not related to that project but has a lot of similar features and alows linking of equipment profiles.
2007/09/17: Decent amount of level 70 gear still missing from program (have to add it via allakhazam piece by piece). Also, the talent trees are incomplete and screwed up (at least for Shaman) so none of the talent/skill point calculations are accurate. :(
Here is the basic screen of WowEquip which looks very similiar to your ingame character screen:
Here is the item selection box, notice you can add or edit new or exisisting items: