For Web writers, the issue of rights can be a sticky one. When writing for magazines or anthologies, many writers keep the rights to their work after giving up the first print rights or first North American rights. I see a lot of writers who are transitioning into Web writing and are shocked at the idea of giving up all rights. But, that's what work for hire is. You are hired to write on a specific topic and the article that results is transfered to the buyer. The writer has no rights in those cases.
Personally, I have no problem whatsoever with giving up rights to an article when I have been paid for it. The only time I don't want to give up rights is when the site or client in question is paying on a profit-sharing basis. I write for several sites that pay based on pageviews or a portion of the revenue generated by pay-per-click ads.
If you want to write for a website and keep the rights to your work, there are several that allow it. Suite101 allows its writers to keep the rights to all of their articles but asks that the articles not be reproduced elsewhere for one year. BellaOnline has the same one-year policy.
I wrote for Triond for a while to test the revenue possibilities. The possibilities are bleak at best. I recently found out that Triond lets its writers keep all rights to the articles they post there. Therefore, I am going to take some of my articles from there and transfer them to HubPages, which has the same rights policy.
If you write for AC, you have the option to publish your work exclusively or non-exclusively. If you choose the non-exclusive route, you can publish the work anywhere else you desire. One way that I've found to make the most of that arrangement is to publish non-exclusive items on AC and then to offer the re-print rights on Constant Content. I have sold several re-print rights there, and though they don't go for much, it's been a nice sideline.