Titanium Ore is the rare ore in Northrend. It can be farmed in Icecrown, Sholazar Basin, The Storm Peaks and Wintergrasp. It shares node spawns with Saronite Ore, so farm Saronite as you fly around and it may spawn as Titanium.
Those who played during Wrath will likely remember that Titanium Ore node that would spawn beneath Dalaran, which would cause many miners to try and fly there first.
Titanium Bars are only used for a couple transmog pieces, but it is needed for the toy Wormhole Generator: Northrend.
You can also farm the following gems and elementals: