well, the first thing is to create a format to save levels.
(for example, storing a list of objects
-type, position, rotation
as a list.

Then maybe write an exporter/importer into a pure textfile.
This way people can at least exchange the textfiles.

To upload it to a server I would try to set up a php (+mySql) solution.
In the JewelDrop Demo Threat I have also posted my simple php
code. Which can upload and download textbased data.
This could be used for levels.
(needs also things like selection of the level to install etc..
but its a beginning)
Good about php is, that you can let it run on a free
online server.


The easiest is though to just let users be able to upload
the level directly as textfiles to your server, and manually
downloading and installing them.
(The most primitive would be to just use a forum, insert
the text-content, and downloaders copy-paste this back to
a levelfile. Its a bit very simple thoug. Better is to set up
a normal file-upload solution.)

You could also put user-level into a level-pack, so people
can download them as a whole pack.