You can list your domains on CAX.com -- http://www.cax.com/submit/ or on sedo.com. I think CAX has a $5 membership fee last time I looked. I think sedo lets you list domains for free, and you can also list them for sale and earn 'parking' revenues from them too.
Sedo.com is usually your best bet, as they attract a lot of end users. Afternic.com tends to attract higher-end buyers. Flippa.com can be used for domain sales, but is more geared to complete websites rather than domain names. NamePros.com is a good place to list if you have decent domains that you are willing to sell at wholesale (reseller) prices (10x less than end user prices).
furryfruit.com is an auction site that you can list domains OR websites on.. Currently they are offering free listings and they don't charge any commission on sold auctions!.. It is a new auction site but you might as well make use of the free listings whilst they get going..