The online tools get their information from the Google data centers, it is "usually" accurate. Depending on how many data centers they use the results can vary.
Personally, I don't bother too much for page rank now but I love to visit mcdar.net/q-check/datatool.asp tool which also tells Google back links.
Not all the time. Sometimes google toolbar displays wrong PR. I mean not accurate. One of my blog have a PR3 but google toolbar sometimes displays a PR7.
I tend to like iwebtool. The last 1 year has never been wrong. But again: it depends on the accuracy of the datacenters.