During the WinHEC (Windows Hardware Engineering Conference) keynote today, Bill Gates announced the release of the Beta 2 for Windows Vista, Office 2007 and Windows Server "Longhorn". Here's a review of the status of some of the major betas available today:

Internet Explorer 7

The next browser from Microsoft, to be available as a download for Windows XP SP2 and included in Windows Vista.
Best link for general information:
http://www.microsoft.com/windows/ie/ie7
Rough timeline: Beta 1 released on July/2005. Beta 2 on April/2006. Final release expected in H2CY06, around the time Windows Vista is released.
Pre-release version available publicly today? Yes, public download from the URL above.
What can I do to evaluate now? Download and install the Beta 2. Follow the IE blog at
http://blogs.msdn.com/ie.

Windows Vista

The next client Operating System from Microsoft, the successor to Windows XP SP2, previously known by the codename Windows “Longhorn”.
Best link for general information:
http://www.microsoft.com/windowsvista
Rough timeline: Beta 1 released on August/2005. Beta 2 on May/2006. Final release expected in H2CY06. Volume license (business) availability in late 2006 and consumer availability (retail stores) in early 2007.
Pre-release version available today for download? Yes, download from MSDN.
What can I do to evaluate now? Download and install the Beta 2 from MSDN. Download the Windows Vista Product Guide (60MB Word doc) from the link above.

2007 Office System

The next Productivity Application Suite from Microsoft, the successor to Office 2003 SP2, previously known by the codename “Office 12”.
Best link for general information:
http://www.microsoft.com/office/preview
Rough timeline: Beta 1 released on November/2005. Beta 2 on May/2006. Final release expected in H2CY06. Volume license (business) availability in late 2006 and consumer availability (retail stores) in early 2007.
Pre-release version available today? Yes. Public download from the link above.
What can I do to evaluate now? Download and install the Beta 2.

Exchange Server 2007

The next Messaging and Collaboration Server from Microsoft, the successor to Exchange Server 2003 SP2.
Best link for general information:
http://www.microsoft.com/exchange/preview
Rough timeline: Beta 1 released on December/2005. Community Technology Preview (CTP) released to MSDN on March/2006. Beta 2 should be available this summer. Final release expected in 2007.
Pre-release version available today? Yes, download CTP from MSDN.
What can I do to evaluate now? Download and install the CTP from MSDN. Follow the Exchange blog at
http://blogs.technet.com/exchange.

Windows Server "Longhorn"

The next Server Operating System from Microsoft, the successor to Windows Server 2003 and Windows Server 2003 R2.
Best link for general information:
http://www.microsoft.com/windowsserver/longhorn
Rough timeline: Beta 1 released on 2005. Beta 2 on May/2006. Final release expected in 2007.
Pre-release version available today? Yes, download from MSDN.
What can I do to evaluate now? Download and install the Beta 2.

As you can probably imagine, more information and other prerelease versions will become available over time. Please check the links referenced in this post for the most up-to-date information.