Membase is a clustered data storage service your application uses.

LevelDB is a persistence library.

That makes LevelDB the kind of thing you plug into membase to get the unique properties it has to offer (or at least for fun).

In fact, the Riak guys are planning on doing exactly that: offering LevelDB as one of the storage back-ends, perhaps even the default.

http://blog.basho.com/2011/07/01/Leveling-the-Field/