When I try to access the tool, the URL redirects back and forth between the Google Account authentication system and the webmaster tools home page. If I try a different browser, it lets me in, so it seems to be a cookie issue for some users.
The page isn't redirecting properly
Firefox has detected that the server is redirecting the request for this address in a way that will never complete.
* This problem can sometimes be caused by disabling or refusing to accept cookies.
Safari can’t open the page.
Too many redirects occurred trying to open “https://www.google.com/webmasters/tools/home?hl=en”. This might occur if you open a page that is redirected to open another page which then is redirected to open the original page.
I can login to all other Google properties without an issue, I am just having this issue with Google Webmaster Tools. I rather not delete my Google cookies, cause that will stop them from tracking me for the past 15 years. :)
Forum discussion at WebmasterWorld.