I've used many. RSS2Blog is pretty good. Takes quite a bit of work to get set up and is a bit pricey if I remember correctly, but has a lot of options and lets you decide what content goes on your sites.
Unique Article Wizard Plugin for Wordpress is ok. I think the plugin is free, but don't know for sure. I find that they have articles on popular topics, but if your site's are tightly themed for a long tail niche, you don't get much content.
Caffeinated Content is another paid one which has better content, but I'm still not in love with it.
Articlebuilder.net requires a paid memebership I think, and it's a bit of manual work, but they will grab articles from article directories and let you export them formatted to post-date your posts in your blog where you just have to upload a file. It'll also export them in RSS2Blog ready format as well.
I haven't used most of these for a while so I don't have all the current info, but there's a few for you to work with
