Rfuse_merged_fs is a project mainly written in Ruby, it's free.
An Ruby fuse (fusefs) file system, which gives ameged view of multiple folders/drives.
Relies on fusefs (latest 0.7.0) gem.
This mounts the file system to the first Argument and the all other arguments are folders to be presented as a merged view.
$ mkdir ~/tmp_fs
$ rfuse_merged_fs ~/tmp_fs ~/Movies ~/Documents