According to Apple's release notes, the free 5.0.1 update includes:
Customise Safari with features created by third-party developers
Find extensions in the Safari Extensions Gallery, accessible from the Safari menu and extensions.apple.com
This update also contains improvements to stability, accessibility and security, including the following:
More accurate Top Hit results in the Address Field
More accurate timing for CSS animations
Better stability when using the Safari Reader keyboard shortcut
Better stability when scrolling through MobileMe Mail
Fixes display of multipage articles from www.rollingstone.com in Safari Reader
Fixes an issue that prevented Safari from launching on Leopard systems with network home directories
Fixes an issue that could cause borders on YouTube thumbnails to disappear when hovering over the thumbnail image
Fixes an issue that could cause Flash content to overlap with other content on www.facebook.com, www.crateandbarrel.com, and other sites when using Flash 10.1
Fixes an issue that prevented boarding passes from www.aa.com from printing correctly
Fixes an issue that could cause DNS prefetching requests to overburden certain routers
Fixes an issue that could cause VoiceOver to misidentify elements of webpages
For detailed information on the security content of the 5.0.1 update can be found here.
Safari Extensions are sandboxed insists Apple, so they can’t access information on a user’s system or communicate with websites aside from those specified by the developer. For increased stability, Safari Extensions run solely in the browser.
Safari 5 also features Safari Reader, which presents single and multipage articles on the web in a new, scrollable view without distracting content or clutter.
The latest version of Safari 5 can be downloaded from apple.com/uk/safari or via Apple's built-in Software Update.