WP-O-Matic is a free plugin. Good but has some issues though. The most complete autoblog plugin is http://www.wordpressrobot.com/features There is no other plugin what does as much as that one.
http://www.blogsense-wp.com! Not free but premium and its my job to develop it so always new stuff coming in. Private community associate with the software too, This community is brand new, but will be full of autobloggers for information exchange. mine is completive with wprobot. cheaper, and has some very nice features that wprobot lacks. Same server image hosting and renaming to include longtail keywords, an rss module, and soon to have link cloaking. also has a private community. wprobot has an articles module which blogsense doesnt have yet, but its coming soon. And wprobot has a more flexible scheduling system. But blogsense has a cooler amazon and yahoo answers module, because the created comments drip naturally over time with blogsense, and doesn't with wprobot. But who knows, this might be changed by now