Homestuck is a webcomic styled as an adventure game, with Flash and interactive segments.
It can be read starting from the front page ( http://www.mspaintadventures.com/ ) and is one of the most prolific comics ever made with over 4000 pages including Flash content, and it updates usually with multiple pages per day.
It follows the story of four kids and later other characters who set out on an epic multiple-universe spanning adventure. Often the best parts of the comic are done in several-minute Flash animations. Here is one from early in the comic which should give you an idea of the scale it reaches:
[S] WV: Ascend from Homestuck Act 2, 'Explore' by George Buzinkai and Michael Guy Bowman
Almost everything in the comic is done by one man, Andrew Hussie, but there is a team of about 20 musicians who create the soundtrack to the comic now consisting of several hundred original tracks.
'Savior of the Waking World' by Toby 'Radiation' Fox
The story gets quite complex but is very rewarding in depth and consistency. It is also very funny. While not necessary to understand the story at all, sometimes it references the previous comics done by the same author, notably Problem Sleuth.
For anyone who is up to date, the next update is expected on 11/11 with Act 6.