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Simple Press has a 'Thanks' plugin that could be used for a type of user rating. There is also the CubePoints WordPress plugin that you can install and then install the Simple:Press cubepoints plugin and it will combine ratings from the blog and Simple:Press that you can setup and display where needed.

Far as a WordPress plugin for user ratings any good one should have some kind of template tag or shortcode you can put in widgets or blog posts. If so you could add that shortcode to a Simple:Press theme where you needed it.