I finally found some working pagerank prediction tool.
I've been using http://www.iwebtool.com/pagerank_prediction , but it is completely broken. It gives you result: "Unable to retrieve prediction at this time".
This website: http://www.seomastering.com/pagerank-prediction.php uses it's own algorithm to estimate your future pagerank based on the quality of backlinks.
It's hard to say how accurate it is, but maybe it at least does something :) This website has got several other useful SEO tools.
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