Ever wanted to check out the first few chapters of your favorite manga series without the terribly time-consuming process of downloading them illegally (or, yeah, buying them)? Check out Weekly Shonen Jump’s official website (Here!), where they’ve posted the first chapters, including: One Piece, Naruto, Bleach, Katekyō Hitman Reborn!, Gintama, Kochira Katsushika-ku Kamearikouen-mae Hashutsujo, To Love-Ru -Trouble-, Sket Dance, Psyren, Nurarihyon no Mago, Toriko, Bakuman., Inumarudashi, Kuroko no Baske, Beelzebub, Medaka Box, Akaboshi, Anedoki, Kagijin, Pyu to Fuku! Jaguar, Wasshoi! Wajimani, D.Gray-man
Some of these, like Kagijin, are brand new whereas others are older. Kochira Koenmae, for example (that’s the nickname for that ridiculously long title) is the longest running serialized manga (others like Golgo 13 have been around longer but did not publish continually).
And if you want manga in English (the above are in Japanese, on the Japanese site) then check out U.S. Shonen Jump’s site (Here!) to get a large selection of their serialized regulars (there seems to be a lot).