Blackfathom Depths: Level 20-27
Gnomeregan: Level 24-33
Maraudon: Level 40-49
Ragefire Chasm: Level 13-15
Scarlet Monastery: Level 30-40
Scholomance: Level 56-60
Stockades: Level 23-26
Stratholme: Level 55-60
Temple of Atal'Hakkar: Level 44-50
Uldaman: Level 35-45
Wailing Caverns: Level 15-21
Zul'Farrak: Level 43-47
I know there are more, but they aren't listed on here so I didn't post them. It would be good to get a definitive list of all instances and what level you need to be to do them to help people decide what instances they want to do.
Any additions, especially Shadowfang Keep as I still want to do this.