Apple also implemented DNS prefetching, a technology pioneered by Google; it's been in Chrome since its launch in September 2008 (try navigating to about:dns). Basically, this technology scans the links on web pages, pre-resolving the IP addresses of the various domains associated with the content behind that link. It does all this in the background.
Just as the iOS 4 adds Microsoft's Bing as an option for the search field, Safari 5 gets the bing treatment as well. Like on iOS 4, the default is still Google, but in addition to Yahoo! you can opt to search with Bing as well. Speaking of Microsoft, Safari 5 also adds hardware acceleration on Windows.
That's the catch-up stuff, now let's look at the big new feature. Safari Reader kind of does what its name implies: it removes all clutter from web pages, displaying only the story. It removes ads and sidebars, stitches pages together when articles are spread out, and presents it all in a printable layout. The ads and various web pages are still fully downloaded, so web sites will still see the correct amount of page views and ad impressions.
Safari 5 is available for Mac OS X and Windows.