If you liked
666 Satan
|
...then you might like
Ao no Exorcist
|
The settings of the stories aren't particularly similar beyond being shounen series, but the heroes are very similar - both are hated for Satan-related reasons and chose their goals to overcome this, and their attitude is pretty similar.
If you liked
Naruto
|
...then you might like
Ao no Exorcist
|
Both are action shounen series, and center around similar characters. Both Naruto and Rin are hated for something that isn't their fault - being the host of the Kyuubi in Naruto's case and the son of Satan in Rin's - and fight to be accepted, both are rather, uhm, dumb, but also charismatic, and both end up getting people to like them eventually.
If you liked
Girl Friends
|
...then you might like
Octave
|
Both are realistic and heartwarming yuri series that show the girls change through their relationships, though Girl Friends is much less serious.
If you liked
Lucky☆Star
|
...then you might like
Tonari no Kashiwagi-san
|
Both are easy to read, have "moe" art and an otaku main characters. Tonari no Kashiwagi-san has more drama and plot, but the comedy scenes have a similar feel to Lucky Star.