Many folks are unaware of other sites.  EquipCompare is one of the mods I use from another host.

http://www.wowinterface.com/downloads/info4392-EquipCompare.html

The only other two I use from other sites are:

Baud Error Frame - http://www.wowinterface.com/downloads/info9825-BaudErrorFrameFanUpdateforWotLK.html

and Chat Bar - http://www.wowinterface.com/downloads/info4422-ChatBar.html

Not sure what the benefits of EquipCompare are over the inbuilt item compare is but, yes, most people do forget that other sites do exist.

Ingame, when I move the cursor over an item or item link, there is a tooltip for that item and that item only.  If the item is "wearable", when I move the cursor over an item or item link, Equipcompare will add the tooltip of the item I am wearing (in that slot ... sometmes 2 extra tooltips) next to the tooltip of the item at the cursor.  It can be confusing and I have bid on items thinking they were the drop and not what I already had on.  Be careful in those cases.

Example#1 - Item Tooltip:

Example #2 - Item Link:

IS this not the same functionality that holding down shift and hovering gives you? They added this to the game a few patches ago with little notification. It took me a few weeks to even realize that it in built in now. Look into your interface options via the ingame menu. There will be an option to change the key used to compare to whatever you choose.

Is this not what you are asking? I apologize if it is not. Also please post back if you find you answer elsewhere or if what I am saying is true so people with similar issues know what works and what does not.

Yeah, this is pretty similar to the equipment compare functionality that was inserted into the UI a few patches ago with the exception that you don't have to hold down shift to make it work.

I personally use tekKompare and it seems to be fine though it hasn't been updated for 3.0.  I like being able to display itemlinks in chat by hovering over them.