Where can I find Growl's log(s)? Would like to use with GeekTool to display it on the desktop.
Growl uses a CoreData database located in ~/Library/Application Support/Growl/
The database is a file called notifications.history
Edit: there does not as yet appear to be any way to communicate with the Growl Control Suite say via AppleScript, to read the database. The functions there are limited. See the Growl AppleScript documentation here
You would have to communicate with the database directly, a database model is included within each build of Growl.