If you own the com, net org versions of the same name and have no plans to use the extras, park them on the version you are using and 301 redirect them to the main domain.
If you just have a few spare domains, put up a 1-5 page site on topic and see if you can get it listed then you can monetize it or sell it.
Park any of the above with a domain parking service and you might make the reg fee each year from type in traffic. Not likely but possible.
If you good keyword domain names, then putting up different sites would be good otherwise if you only have different extensions of the same name then use 301 redirect
You're advised (on the google webmaster info) to not duplicate your website content. So as dman_2007 says, redirect them with a 301 redirect if they are just different domain extensions. Otherwise, you could create a mini site with some links to your other site. I did this for a while with a handful of sites (e.g. www.board-shorts.co.uk) but you have to promote those a little then!
