The Electronic Journal of Combinatorics have these "Dynamic Surveys" which are allowed to be updated -- some of which are quite massive.
EDIT: It has been pointed out to me that there is another category, called "articles", in the Electronic Journal of Combinatorics which include non-dynamic surveys and exposition papers. At the moment, there are not many such articles, but there are some (I counted 14 in total).
Research articles as well as surveys and articles of more general interest are welcome.