Not sure if it works with MACs but SUPER will convert pretty much anything to anything. There is a list on the link quoted and m4v is mentioned as a supported format.

There is stacks of free storage space around - I used AOLs X-drive for a while which offered 5gb free and currently use ESNIPS which offers 5gb of free space. Three options on it: Private, Shared with named group, shared with everyone. I use it to store files and music and all sorts of stuff on. It will stream video and sound if you want to share things from there and allows you to embed into a site etc if you want - Video is an example of the only video I have uploaded there at the moment; there's an example of a Song here.