If you liked
Rosario to Vampire
|
...then you might like
In Bura!: Bishoujo Kyuuketsuki no Hazukashii Himitsu
|
Vampires, ecchi, harem - the mangas have many similarities while being quite different. Rosario+Vampire is better plotwise and more serious as a whole while In Bura! is more "light".
If you liked
Velvet Kiss
|
...then you might like
Ojousama wa Nigedashita
|
Velvet Kiss and Ojou-sama wa Nigedashita are so similar they're almost identical (plot-wise).
If you liked
Claymore
|
...then you might like
Koukaku no Regios
|
Claymore and Chrome Shelled Regios have many common things - like special warriors fighting monsters to protect humanity. The works are really similar and give off the same feeling. Chrome Shelled Regios isn't as dark as Claymore, but you get pretty much the same feeling from it. By the way, you'd better watch Chrome Shelled Regios's anime version if you want to get something similar to Claymore's manga.
If you liked
Break Blade
|
...then you might like
Shingeki no Kyojin
|
There is a big similarity in the position the protagonists are put in (won't say more so I won't spoil it for you). Both manga give the same feeling of a war for survival against seemingly superior enemies.