svn.complete.org supports both standard (http) and encrypted (https) sessions. You are free to choose which you use, unless you are making commits, in which case you MUST use https. Most people can just use http. The basic format of a command is this:
svn co http://svn.complete.org/repository/project/head -d localname
The repository and project specify the appropriate names as listed, and in some cases will be the same. The localname specifies the name of the local directory. An example command would be:
svn co http://svn.complete.org/offlineimap/offlineimap/head -d offlineimap
You can also check out previous releases by using paths other than "head".