Yaoi titles: Ore ni Koi Shite Dousunda (nephew falls for the womanizing uncle who is looking after him), Ohana to Chocolate (orphan falls for adoptive guardian), You and Harujion (another orphan romance), Ouchou Haru no Yoi no Romance (warning: underaged character!), I'm Aware of Love at Night (boy falls for butler who pretty much raised him), Gerard and Jacques (butler falls for master who looked after him since childhood), Vassalord (vampire loves the Sire who turned him and brought up) and Yume no Kodomo (boy falls for his sister's friend who gives him shelter).

For a VERY dark version of the same theme: Zion no Koeda, where the ward grows up to have a dark obsession with his guardian, and basically keeps him prisoner.

Possible shoujo titles: Dengeki Daisy (girl falls for protector that she co-habits with), Kanon (girl falls for violinist who takes her under his wing).

Full Moon o Sagashite ! --i can't believe no one said it yet

Ai Hime - Ai to Himegoto
Lovesick

Shiawase Ikura de Kaemasu ka?

New shoujo title that probably qualifies: Nocturne (PARK Eun-Ah). Manhwa, though.

sexy guardian
Arakure

here are some other titles that handle that theme( be warned they're not shoujo or yaoi)

FullMetalPanic
Happy World (guys falls for his guardian angel, really
complicated though)
Kurenai
flame of recca

oh yeah can't believe i forgot this one
black cat

Words to live by: "Let sleeping dogs lie"
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oh dengeki daisy is brilliant and really good if you like a bit of mysterious drama. Shamelessly advertising here- but check out the mangaka's other works as well.

Grand sun is really good too, although the twists at the moment are getting a bit weird.

sugar family is really cute!

hmm, i reckon if you checked out mangas with bodyguards maybe? or even butlers like Shitsuji-sama no okiniiri

"Fashion is nothing but an induced epidemic." George Bernard Shaw