We use many, many things from Dragon. A feat here, a spell there, a weapon elsewhere, numerous monsters, and so on.
The ones I always keep handy as a player:
-Power Fantasy: Action Movie Stunts in D&D from #306
-Swashbuckler from #301 (mainly for the new skill uses)
Ones I've used frequenly as a DM:
-Annual 5, for the monsters of Tharizdun (although many have been reprinted since)
-#306, for the battleloths, elemental demons, and hellforged devils
-#300, for the planar dragons (until the Draconomicon reprinted them)
-#321, for the rest of the planar dragons
-#278, for the stonelord (until it was reprinted in Complete Warrior)
-#310, for the specialist fighters
-#309, for the Incursion info
-#315, for nearly everything