Audit Trail: Now you can track all the changes made to your sites by different users.
Sites Robot.txt and Fav Icon: With this release you will be able to set a robot.txt for you force.com site which enables you to better control how search engines should index your site’s pages. Fav Icon is also a useful way of personalizing the browser in showing an icon next to the title of your website (in tabs) and also in favorites/bookmarks.
Standard Pages: Allows you to bring selected standard pages and expose them to the public via your force.com site (suitable for ideas page, etc).
Sites Packaging: With this feature we can package force.com sites, apex code, etc and install them into other instances.
Force.com Web Service API Improvements
Improvements in the timeout limits of the callouts: This is potentially good when the code takes a longer time to process and bring back the results. Now we can configure it in a way to have extended timeout periods resulting in more run-time stability.
Also Wider range of WSDL types are supported by Spring 09 release.
Database and Reporting
Summary Report Data Snapshots: Previously data snapshots were available on reports, however, with this release you can use snapshots across large amounts of data, where only the summaries will be stored, as well as create historical views of metrics, KPIs and other summary data.
Data Grouping Functions: This is actually a very useful upgrade, now you can group the entries of a report to better analyze aggregate data.
Delegated Data Administration: This feature allows you to grant permission to non-admin users to report, search, modify, transfer, share, unlock, or approve any record regardless of sharing settings on an object basis.
Dashboard Finder: This enhancement allows you to search through your dashboard items (a real time-saver!).
Apex Code Auto-Complete in Force.com IDE: Finally! When you are coding in Eclipse (force.com IDE) auto-complete feature will be available to assist you with writing the code.
Enhanced Page Layout Editor: This one is my favorite enhancement, using this feature now you can create much better layouts. You can drag and drop fields and items around, “save as” a layout, leave a spot empty (blank spaces), design the mini-layout, etc.
Cross-Object Workflow: With this feature you can update a field on a parent record within a workflow!
Access to Email Logs in the Application: This report shows you whether your outbound emails were received or bounced back and basically gives you a comprehensive log of your outbound emails.
Mobile Web Tabs for iPhone You can now create mobile-ready Visualforce Tabs for the iPhone. This enhances the client with a near-limitless method of extension – enabling you create content for the iPhone using the Visualforce user interface framework. The hybrid iPhone client that supports this functionality will be released next month.