There are hundreds of third party PageRank tools that show Google's Toolbar PageRank score. Some of the more popular ones can be found by doing a search in Google for pagerank checker.
Many of these tools are now not able to produce an actual PageRank score for a site. This even goes for my PageRank prediction tool that was designed as an April Fools Joke in 2004.
Did Google intentionally break what was working for third party application to check PageRank? Was it an accident? Is something broken at Google?
It does not appear to be an issue with every tool. But it is impacting many tools.
How does one get the PageRank toolbar score? More on that with these articles:
Forum discussion at DigitalPoint Forums.