Apple has had a Safari 4 Beta out for a few weeks now. Should you wish to try it out on your Mac, a word of warning: installing this version utterly messed up my Mail, in particular my IMAP accounts. I blew away all traces of the accounts and recreated them from scratch in Mail. That didn’t help, nor did any of various other cleanup operations. Uninstalling it made my problems disappear. It is possible this problem is specific to me (such a critical problem would have raised louder howls!) or I am misdiagnosing it (one is a browser and the other a mail client). So usual acronyms apply: YMMV, FWIW.

Some interesting new features in Safari 4:
- Offline support (HTML5)
- New JavaScript engine (Nitro)
- Cover Flow for bookmarks and history
- “Top Sites” — new tab options like Google’s Chrome
- Tabs on top like Google Chrome (what where they thinking?!)
- A JavaScript debugger and profiler
Available for Mac OS X and Windows, if you want to brave it!
UPDATE: The problem was incompatibilities between Safari 4 and one of my Mail Plugins (Proxifier), which I was not using anyway. I removed /Library/Mail/Bundles/Proxifier
and all is well!