Mainstream

• something that is not out of the ordinary or unusual;
• something that is familiar to the masses;
• something that does not belong to an identifiable genre, such as detective fiction, horror, fantasy, or science fiction.

Antonyms:

• underground (culture),
• off-beat[?],
• eclectic[?] (although an improper use of the term),
• avant-garde,
• special[?] (in the context of education, where it is often used as a verb).

Related:

• blogstream

See also normal.