Just a quick post to let folks know about a free iOS Tenpai trainer tool (available in the US and Japan Apple/iTunes store). It's simply called Tenpai and while it's not fancy, it is useful. It gives you a random hand and by default hints are off (the vertical column of numbers above the tiles). Your goal is to get your hand into Tenpai as quickly as you can - discarding what you want and drawing another random tile. You get 15 turns to get into Tenpai and make your hand ready.
Normally the hints are off - but you can see the numbers above indicate how close to various Tenpai conditions (four sets and a pair, seven pairs, 13 orphans) you would be if you discarded that particular tile.
It's worth the free price - I find myself practicing without hints to see how quickly I can bring my hand into the ready state.