Microsoft has a beta site [beta.microsoft.com] where you can use your Passport account to sign up to beta test programs. For example, if you wanted to try out the upcoming OneCare [webmasterworld.com] service, this would be the place to sign up.
I signed up to the beta program about a year ago. I signed in as a guest then selected areas I was interested in. It was about 6 months later that I received an email invitation to download and evaluate some software.
here is a more complet list of the products you can sign up for:
* Baseline Security Analyzer 2.0--MBSA20 * BizTalk Server 2006--BizTalkBetaTeam * Business Scorecard Manager 2005--MaestroTester * Command Shell ("Monad")--mshPDC * Device Emulator Version 1.0--MSDEVICE * IIS Debug Diagnostics 1.0--DebugDiag * iSCSI Software Initiator 2.0 ("Osaka")--iSCSI * "Longhorn" Developer Platform--longhornsurvey * Microsoft Management Console 2.1--MMC21Guest * MSN Friends and Family Testing Opportunity--BetaFandF * MSN Mobile--msnmob * MSN Remote Record Service--myshows * Network/Connectivity Device & Driver Testing Suite--NDISTest * Office 12--O12Guest *NEW* * Office Communicator Web Access ("Budapest")--MOCclient * Office Live Communications Client--LCclient * RACF Management Agent--MABeta * Roaming Favorites--RoamFavs * Small Business Software--smallbizsolutions * SQL Server 2005 ("Yukon")--sql05beta * SQL Server Migration Assistant for Oracle--SSMA_test1 * System Center Data Protection Manager 2006--dpmbeta * Systems Management Server 2003 SP2--SMSSP2Invite * Virtual Server 2005 SP1--vssp1BetaTester * Windows 2000 Update Rollup Package--Win2kURPTest * Windows Driver Foundation--Guest4WDF * Windows (Package) Installer 6.2--WinInstBeta * Windows OneCare--OneCare * Windows Server 2003 "R2"--wInr2nOm -or- R2B2Beta * Windows Server Update Services (WSUS) Open Evaluation Program (OEP)--WUSOEPPart * Windows XP Media Center Edition?--MS_BetaApp