Well - completely different in tone and theme is the other series Lois Lowry is famous for, the Anastasia Krupnik series. They (especially the first one) are really good, very funny, witty and clever; if you are expecting serious, dystopian stuff like in the Giver/Gathering Blue you'll be disappointed, however.
From her more serious books, I remember liking "A Summer to Die" and "find a stranger, say goodbye" but I haven't read them in a long time. Both are pretty sad.
If it's the theme and tone of the Giver you like, you might want to check out some non-lois lowry books that are very similar:
This Time of Darkness by HM Hoover (young adult/sci fi) - it's very good, about 2 kids who try to escape an underground city where reading and learning are forbidden and the citizens are told the outside world no longer exists
Anna to the Infinite Power by Mildred Ames - a little dated (it came out in the 80s and the technology stuff is out of date) but still really good YA/sci fi novel about a selfish girl who discovers she's a product of a creepy, top secret government experiment
The Handmaid's Tale by Margaret Atwood - an adult (readable for teens) novel about a future where society has been restructured in a controlling way because very few people can have children. creepy, sad, thoughtful.
I hope you enjoy, I love lois lowry books too. there are so many it is hard to know where to start (and most of them are very different from the "giver" trilogy.)