This most recent one was a briefing paper, prepared Jul 21st 2002 before a meeting on Jul 23rd 2002.
The Downing Street Memo is the minutes or notes from that meeting - ie what was said by the participants.
This new one is the information they were given as a "briefing", to make sure they were up to speed with the topics they would be discussing.
Additionally, six UK Cabinet papers were leaked last September. Those date from earlier in 2002, and include reports by British diplomatic staff on their meetings on 13th and 17th March with Condoleeza Rice and Paul Wolfowitz, as well as discussions of the practicality and legality of war by UK Foreign Secretary Jack Straw and others. For those papers, see
http://cryptome.org/leaks-brief.htm and a thread on here at
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=104x3809302#3823156(that last thread seems to have provoked the resurgence of interest in the earlier six)
So that's 8 documents in all:
(1) Downing Street Memo:
minutes of Jul 23rd meeting
(2) Briefing Paper (start of this thread)
document prepared to inform participants of above meeting beforehand
Earlier leaks:
(3) Iraq: Options Paper, prepared by the Overseas & Defence Secretariat in the Cabinet Office, dated 8 March
(4) Iraq: Legal Background, prepared by the Foreign & Commonwealth Office Legal Department, dated 8 March
(5) Memo from Sir David Manning to the Prime Minister, dated 14 March
(6) Memo from Sir Christopher Meyer to Sir David Manning, dated 28 March
(7) Memo from Peter Ricketts, Political Director, Foreign & Commonwealth Office, to the Foreign Secretary, Jack Straw, dated 22 March
(8) Memo from Jack Straw to the Prime Minister, dated 25 March
All 8 of these documents first appeared in stories by Michael Smith, the earlier six in the Telegraph and the latest two in the Sunday Times.
(Edit: Telegraph story that broke the first six:
http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2004/09/18/nwar118.xml )