In addition to the normal Dungeon set 3 armor sets, Blizzard put in several alternate sets meant for variation. All of the items are available in various 5 man instances in the Outland's, although most of them are heroic instances. These sets are alternate sets for various classes depending on your spec, they may or may not be better than your base set.

If nothing else they are a challenge to collect since they are mainly found in Heroic instances.

You can find all the details here: Dungeon Set 3.5 Information (The Alternate Sets).