The first one I encountered was Carnivalesque, which focuses on ancient, medieval, and early modern topics. (a_d_medievalist is currently hosting its eleventh installment.)
There's a twice-monthly history carnival. And a poetry carnival. One of the older carnivals is the Philosopher's Carnival, which was, in turn, based on Carnival of the Vanities and the twice-monthly Tangled Bank science carnival.
Those are the academic ones I'm aware of, but the phenomenon is hardly limited to scholars. The Blog Carnival Index lists huge numbers of them, including cats, dogs, expats, and music. As with every project, the challenge will be maintaining the quality of these projects in the long run, of making them something more than just more annotated lists of links, maintaining their usefulness as insightful guides to a given fortnight or two out there on the world wide web.