Genshiken - Mainly about an otaku club, just hanging out. It's a slice-of-life/comedy. Many anime clubs at schools like to copy Genshiken.
Hidamari Sketch - About some girls living in an apartment in front of their art school. It's a slice-of-life/comedy. You might not like the art style, but just recommending.
Natsume Yuujinchou - A supernatural slice-of-life. Very good. It's about a boy who is a loner because he can see ghosts. He inherits a book from his dead grandmother who had the same power as he did. The book contains the names of spirits that she got and once they lose their name, they become servants. He wants to free the spirits from the contracts. The anime is mainly about Natsume and how he is an outcast become of his ability. However, as the story progresses, he starts making friends with humans and ghosts. The story behind some of the ghosts are sad and more and more gets revealed about his grandmother and how she captured the ghosts.
Mushishi - Not really a slice-of-life. It's a supernatural anime about creatures called Mushi and Ginko, a Mushishi who goes around investigating it. It's an episodic anime, each episode features a different type of Mushi. Some (well most) stories are sad (like the man and woman who cannot escape a bamboo forest) and some just makes you feel good (like the guy who got infected by a Mushi and goes around chasing after rainbows).
Great Teacher Onizuka - One of my favorite comedy animes. It's about a teacher who used to be a gang member and he teaches a bad class that tries to push him away. He tries to make the student like him by all means and becomes the greatest teacher in the school. It's a pretty old anime from like 2000, art style not that great but somehow I just marathon all the 43 episodes.
Seto No Hayanome - Pretty funny. Give it a try.
Aria The Animation - Wonderful peace of animation and a very peaceful slice-of-life anime. The artwork is just real beautiful. Watch it alone in a bright lit room.
I'll think of more later.