Negima!?, Sayonara Zetsubou Sensei Recommendations
Both shows are from the same studio, SHAFT, so they share quite a bit in terms of humour and style. Zetsubou Sensei has much more dark humor though, and a more distinct artistic style (in some ways similar to xxxHOLIC). Negima is a bit more random, and focuses a little more on magic and jumbled relationships and such.
The alternate version of Mahou Sensei Negima!, Negima!?, has a lot in common with zetsubou sensei, it has various parodies to other animes, randomness, and at times, cold humor
Harems where the teacher is the guy and all or the majority of the class is girls.
I know your not suppose to recommend sequels or related series but I think this is a special case because Shaft did the animation production for both Sayonara Zetsubou Sensei and Negima!? which gives them a similar feel.
Just watch the opening theme for the two series, you will see what I mean.