Plone 2.5.3:

Plone for Windows (20.5 MB) - includes Zope 2.9.7 and Python 2.4.4 for Windows 2000/XP/2003
Link: http://plone.googlecode.com/files/Plone%202.5.3%20%28Zope%202.9.7%29.exe

Plone for Windows (17.5 MB) - includes Zope 2.8.9 for Windows 2000/XP/2003 (only use this version if you already have a Zope 2.8.x-based site)
Link: http://plone.googlecode.com/files/Plone%202.5.3%20(Zope-2.8.9).exe

Plone for Mac OS X (38 MB) - Intel Installer (Includes everything you need)
Link:
http://downloads.sourceforge.net/plone/Plone-2.5.3-Installer-OSX-Intel.zip?use_mirror=superb-west

Plone for Mac OS X (38 MB) - PowerPC Installer (10.4 only - Includes everything you need)
Link: http://downloads.sourceforge.net/plone/Plone-2.5.3-Installer-OSX-PowerPC.zip?use_mirror=superb-west

Plone for Linux (31 MB) - Unified installer - for Linux/BSD/Mac OS X/UNIX/Solaris (compiles and installs Python, Zope and Plone plus dependencies for you)
Link: http://downloads.sourceforge.net/plone/Plone-2.5.3-UnifiedInstaller.tgz?use_mirror=osdn

Plone for all platforms (8.2 MB) - Product package (requires Zope 2.9.7 or Zope 2.8.9+Five already installed)
Link:  http://plone.googlecode.com/files/Plone-2.5.3-final.tar.gz

Plone 3.0 (Beta release 3):

Plone 3.0 (Beta release 3) for Windows (22 MB) - Beta 3 installer (Includes everything you need)
Link: http://plone.googlecode.com/files/Plone-3.0-beta3.exe

Plone 3.0 (Beta release 3) for Linux (22 MB) - Beta 3 Unified Installer - for Linux/BSD/OS X/UNIX/Solaris (compiles and installs Python, Zope and Plone plus dependencies for you)
Link: http://plone.googlecode.com/files/Plone-3.0-Beta3-UnifiedInstaller.tar.gz

Plone 3.0 (Beta release 3) for all platforms (10.3 MB) - Beta 3 Product Package (requires Zope 2.10.3 and Python 2.4.4 already installed)
Link: http://plone.googlecode.com/files/Plone-3.0-beta3.tar.gz