Here’s a list of components developed for Windows Vista or during the same development time frame and included in Windows Vista, that have also been made available for installation on Windows XP:

Component Notes
Windows Internet Explorer 7.0 Windows Internet Explorer 8.0 is now in beta
Windows Media Player 11  
Windows Defender 1.1  
.NET Framework 3.5 Redistributable download
.NET Framework 3.0 Included in .NET Framework 3.5
Windows Search 4.0  
Remote Desktop Connection 6.1 RDC 6.1 also included in Window XP SP3
Windows PowerShell 1.0 Windows PowerShell 2.0 in beta
Image Mastering API v2.0 API for burning images to CD/DVD
Windows Script 5.7 Release notes
Link Layer Topology Discovery Responder makes Windows XP machines visible in the Windows Vista Network Map (KB922120)
Compound TCP (CTCP) support  
Read-only Domain Controller Compatibility Pack Required for Active Directory support only
Network Access Protection Agent Included in Windows XP Service Pack 3
Kernel Mode Cryptographic Module Included in Windows XP Service Pack 3
Black hole router detection Included in Windows XP Service Pack 3
No product key required during setup Included in Windows XP Service Pack 3
Windows Live Photo Gallery Based on Windows Photo Gallery
Windows Live Mail Based on Windows Mail
Microsoft Management Console 3.0 Included in Windows XP Service Pack 3
Silverlight Based on WPF; Silverlight 2.0 is in beta
Virtual PC 2007 Originally slated to be a feature of Vista called Virtual PC Express
Windows Remote Management An update is available that supports the latest ws-management standard
Network Diagnostic Tool Replicates some of the network troubleshooting functionality available in Windows Vista
Peer Name Resolution Protocol (PNRP) version 2.0 Included in Windows XP Service Pack 3
Windows Imaging Component Includes RAW and Windows Media Photo codec
Windows Rights Management Services Client with Service Pack 2  
XML Paper Specification Essentials Pack  
Microsoft Color Control Panel Applet for Windows XP Not a direct port but Vista includes a Colour Management applet in Control Panel
Microsoft Core XML Services 6.0 Included in Windows XP Service Pack 3
CredSSP Security Service Provider Included in Windows XP Service Pack 3
Ink Analysis API Windows XP Tablet PC Edition only