There are a number of versions of Seagull available for download, pick the one that best suits your needs. Following the links will take you to a SourceForge mirrors page, simply choose the geographically nearest location to where you are to be taken to the appropriate mirror. Detailed install instructions are available at the wiki.
You'll want a release from the stable branch if you're using the framework for customer sites or are in a similar situation where reliability and stability are important.
A full list of historical releases is also available.
The download package provides the smallest amount of downloadable code and just the core framework components. Currently it's distributed with PEAR and FCKeditor wysiwyg libraries. Without said libraries the core Seagull code is just 504kb, we're working on further extracting the dependencies. Also in the near future the user module will be extracted and installable on demand like the other modules.
The full package is available from svn, minus a few administrative scripts and non-essential examples. If you're a developer wanting to run the test suite, or you want to try the many modules/applications provided with the framework, this package is your best bet.
The PEAR package is recommended for experienced developers and offers the advantage that it fully integrates Seagull with your PEAR repository, and takes advantage of any libs you may already have installed. Obviously when you choose this option you can upgrade your PEAR libs independently of when new Seagull releases come out. The PEAR installer also resolves all package dependencies automatically.
Please consider the following versions for your database and webserver software to get the best results from Seagull: