Dog Days'

Both shows kind of having the same plot in the beginning. The summoning of human from another world. However, in Zero no Tsukaima, there's more ecchi scenes and the battles might get tense and serious. Unlike Dog days', where the battle are more entertaining, enjoyable, and joyful. 

People come from a different world and fall in love and stuff... 

Both characters are cute. Although both plots are different, they both have a sense of fun-loving, and the show include slight comedy, and both shows are very carefree. Moreover, both shows are targeted at the same age group. Hence, this two shows have quite a bit of similarities.