Wordpress: You get the power of manipulating the Server Side freely. Think plug-ins, widgets, Ajax... You host it yourself.
Blogspot: You only have JavaScript and few Server side commands. Good for simple blogging. They host it for you. Secure hosting that handle effectively massive traffic.
Others (Like posterous, tumblr...): You are limited, some how. I prefer to stay away from them, as they may surprise me with new rules.
You can get a free account on all these. There's also blogsome and livejournal, but not as good as the above.
I've used and demoed them all. Your mileage may vary.