This past week I was working with a client of mine to determine their upgrade plans over the next couple of years.  Since the majority of their environment is Microsoft, we looked for a good roadmap from Microsoft, but came up with nothing current.  So I went to a few of the product homepages and came up with the following list, which I think is helpful:

Exchange 2010:  due out the second half of this year.  Currently in public beta:
http://www.microsoft.com/presspass/press/2009/apr09/04-15Exchange2010PR.mspx

Windows 7:  Currently in limited beta, Release Candidate is due out next week.  No official release known yet, but it could be within 3 months:
http://www.microsoft.com/windows/windows-7/default.aspx

Office 2010: Previously known as Office 14.  No official release known yet, but Balmer has stated that it will be released in 2010:
http://en.wikipedia.org/wiki/Microsoft_Office_14

Windows Server 2008 R2: Currently in Beta, due out in 2010:
http://www.microsoft.com/windowsserver2008/en/us/R2.aspx

SQL 2008:  Currently available:
http://www.microsoft.com/sqlserver/2008/en/us/default.aspx