I've written on my own blogs, on Yahoo! Voices (formerly Associated Content), Squidoo, and HubPages.
By far, my own blogs have been the most profitable. I had a blog on digital photography that was making a few hundred bucks per month, but that dream was killed by Google's Penguin update.
On Yahoo! Voices, I make $30 to $50 a month. But my account is about five years old, and that's a large portfolio (150+ articles, with about 10 to 20 serious earners).
My HubPages earnings are much lower, but I also just started writing here a month or two ago. I plan on writing some more here, and I can see making double digits in the next few months ($25, maybe $50).
It seems to me that the highest earning potential is going to be your own blog / website, although the topic matters to some extent. I've got a blog that makes about $8 to $10 CPM from AdSense alone, which blows both Yahoo! Voices and HubPages out of the water.