There are many different pre-Beta versions of Windows 7 (called builds), many of which are internal Microsoft releases used to build improvements upon. This list is used to help identify any versions that have been mentioned in the news/blogs:
Milestone 1 (Build 6519 and 6574) - The first known Windows 7 build released to partners in Jan 2008. A second M1 release was identified as 6574 in a leaked video. [First review of 9519 here]
Milestone 2 (Build 6589) - M2 was demonstrated at the D6 conference and included the first changes to the taskbar.
Milestone 3 (Build 6780) - This version was released in Sept 2008 and included ribbon bar changes to some applications, but remained similar in appearance to Vista.
Build 6801 - Most current users of Windows 7 will be using this version, distributed at the Microsoft PDC.
Build 6933 - This version of Windows 7 was shown at PDC but not distributed. It contains the new taskbar which may appear in the final release
Build 6936 - Screenshots of this build have appeared around the net as leaked images.
Build 6948 - Official Windows blogs have released further screenshots identified with build 6948.
Build 6954 - Rumors suggest that build 6954 contains a new animated bootscreen which OEMs can customise.
Build 6956 - This version of Windows 7 was leaked around 6th December 2008, appearing as a VHD from a Chinese Windows conference.
Build 7000 - Beta 1 of Windows 7, released to the Technet and MSDN subscribers in early January 2009. [Review here] Released to general public on 9th Jan 2009 with filenames:
32 Bit : 7000.0.081212-1400_client_en-us_Ultimate-GB1CULFRE_EN_DVD.iso
64 Bit : 7000.0.081212-1400_client_en-us_Ultimate-GB1CULXFRE_EN_DVD.ISO
Build 7022 - A popular leaked build that includes some slight performance improvements over Beta 1. IE8 is updated to RC1, the pain application has changed and there are some icon changes.
Build 7025 - This build appeared on a Chinese blog [here] and appears to contain additional features, such as edition selection (Starter, Home Basic, Home Premium, Business and Ultimate).
Build 7032 - Compiled on January 29th 2009, this build seems to be a minor change over previous releases.
Build 7048 - This build incorporates some major changes over the Beta 1 release, such as a new Start Menu orb, backgrounds and sounds, plus expanded control over which core components are installed (such as IE8 and WMP).
Build 7057 - Compiled on 5th March 2009 at 8PM, this release includes a new logon screen and security fixes to the UAC system. New wallpapers and user icons are also included, along with minor bug fixes.
Build 7068 - Compiled on 21th March 2009 at 1:22PM, this release includes a larger driver database plus some minor performance tweaks
Build 7070 - Screenshots of this version apparently leaked via the son of a MS employee through an internet forum.
Build 7100 - Release Candidate 1 of Windows 7.
Build 7200 - Release Candidate 2 of Windows 7.
Build 7600 - RTM build of Windows 7 - this will be released to TechNet/MSDN/Partners soon.
Please Note : There are many other builds that have been leaked, but these will not be added unless they are a major update to the latest RC builds.