Magento: Blank page in Connect downloader
Magento is extensible with plugin modules. Many free as well as commercial modules are available through the Magento Connect marketplace. Extension packages bought from the marketplace are installed through the Connect Manager. Unfortunately, having Magento working properly doesn't automatically mean the Connect Manager also works. It may complain about not having sufficient write permissions on the Magento folder or even worse, simply displays a blank page.
Blank page: Missing Dependencies
There are several PHP extensions that must be installed into your PHP installation to allow Magento to work. There is a list in the System Requirements page, as well as an automated check. While these extensions are sufficient for getting Magento to work, the Connect Manager has some extra dependencies. For that, you will also need the extensions php_zlib due to a call to gzcompress and php_ftp because of ftp_connect.
This can take you a long time to figure out, as there won't be any error messages, not even in the error log. This is because Magento (in a display of bad design), calls those functions with the error-suppression operator @ in front, without checking whether they actually exist or at least documenting the dependencies.
Insufficient Write Permissions
To be able to install your downloaded extensions, Connect Manager will need write access to the Magento installation. One way to accomplish this is by making the entire installation world-writeable. The security implications of this send shivers down my spine and I prefer to let the Connect Manager use the FTP credentials for the user that owns the Magento folder.
Even if you set these up, the downloader will still complain about not having sufficient write permissions. This is because not all directories that it needs to access are created automatically or with the correct permissions. Make sure the following folders exist within the Magento installation folders and are writeable by the FTP user: