For in-game, there is a mod (should be downloadable from Curse.com) called BonusScanner.
It shows how much important stats would go up or down when swapping in peices of gear, taking into account talent bonuses as well.
It adds a couple of lines to the item tooltip.
It also shows item level. I have found it very handy.
Not sure how recently it's been updated, but I can confirm the latest version works with the recent 2.4.2 patch.
<edit> Oops. I got my mods all mixed around. It's not BonusScanner at all. It's called RatingBuster. Pick it up, it's awesome.
Edited, May 23rd 2008 3:12pm by Fraeky