There are custom scenes on this forum although most of the better ones have been included in the disto of R4.
There are many new ones in my latest set here:
http://www.rabidhamster.org/phpBB2/viewtopic.php?t=539
but you have probbaly seen that thread already and it is for high spec machiens and there are a couple of performance issues that me and Gordon need to sort out for total smooth running.
For creating your own some knowledge of programming is generally required. If you have a specfic questions then just ask here and we will do the best to help you out.