Google pagerank predictor from RustyBrick is a very handy tool which allows you to check your future PageRank.
Rustrybrick is an SEO company which provides a wide range of services, and this pagerank predictor tool is presumably a result of their specialised knowledge, although there is no indication of how exactly it works.
They say that it attempts to determine a future Google PageRank value of a particular site.
Although the site says that it is for entertainment only, I have been using it for a few months now and have found that it does seem to predict the movement of this site's PR quite well.
You do need to use it on a regular basis to chart the movement of your site's PR but it could be used as an extra tool in your SEO to get an idea of how any changes to your site may affect your PR.