The SVN Repository, or Subversion, is a secure, online version control system used to manage files and source code over time. The files are stored in a central server and accessed via the Web. The repository is much like an ordinary file server for storing and transferring files, except that it also remembers every change ever made to your files and directories. This allows you to recover older versions of your files and examine the history of how and when your data changed. Files stored in an SVN repository can be viewed through a web browser (read-only) or viewed and edited through a free client download, such as TortoiseSVN.

Benefits of SVN:

Directory versioning: With Subversion, files and directories are versioned to track changes.
True version history: Subversion tracks who, when, and what changes have been made to the file or directory.
Protection against disk crashes: Subversion implements a backup system to prevent loss of files.

Read the Subversion Book here for more information about Subversion.
Here's an article from redhat.com explaining why Subversion is better than CVS.

TortoiseSVN

Subversion has a Windows client, TortoiseSVN, which is extremely well integrated with Windows Explorer. TortoiseSVN allows customers and professionals to easily synchronize files between their local machines and the central repository. You can use the TortoiseSVN client to checkin a file by an upload, checkout a file by downloading, and to view file history.
Features of TortoiseSVN/Subversion

Windows Shell (Explorer) integration. Can be used without any IDE.
Icon overlays show immediately which files/folders need to be uploaded (committed) to the database.
Easy to use dialogs for version control operations.
The UI is available in different languages.
Global revision numbers. You can get old versions of your project and not just single files.
Atomic transactions. Interrupted operations don't affect the repository at all.

System Requirements

Win2k SP2 or WinXP or later, IE5.5 or later

You can find a MAC client named SmartSVN at SmartSVN - Subversion/SVN Client

Using TortoiseSVN with Pligg

Getting the latest files from Pligg SVN:
1. download TortoiseSVN or another SVN client
2. If using Tortoise (not sure about other clients) right click on a folder where you want to download pligg and go to tortoisesvn > export
3. use this url if you're not using command line https://pligg.svn.beanstalkapp.com/pligg/
that will get you the latest up-to-the-minute code.

SVN Twitter

You can keep track of the latest changes by viewing or subscribing to our Pligg SVN Twitter feed. The Twitter feed displays snippets of information about our latest SVN revisions whenever we commit a change to the Pligg SVN. Use the url below to view the Pligg Twitter page:
Twitter / pligg