1. Confluence 2.6.1 or later.
a. See http://www.atlassian.com/software/confluence/ to download
2. Windows SharePoint Services (WSS) v3 (free with Windows Server 2003).
To check whether Windows SharePoint Services are installed, go to Start, All Programs, Admin, SharePoint 3.0 Central Administration. If you can see 'SharePoint 3.0 Central Administration', it is already installed.
a. See http://office.microsoft.com/en-us/sharepointtechnology/FX100503841033.aspx?ofcresset=1 for information.
b. To download, see this page.
3. (optional) Microsoft Office SharePoint Server (MOSS) 2007 (Standard or Enterprise)
MOSS is Required for the following features:
a. Integrated Search
b. Single Sign-On functionality from SharePoint using Windows Integrated Authentication to Confluence using Forms Based Authentication. See this page for information.
4. To download a trial version of MOSS, see this page.
5. To determine whether you have WSS v3 or MOSS 2007 installed, go to Add/Remove Programs. If you see "Microsoft Office SharePoint Server 2007" you have MOSS; if you see "Microsoft Windows SharePoint Services" you just have WSS v3.
See also a review of the SharePoint and Confluence Connectivity Options.