I've never used python before. I've used php for about 5 years now. I plan to learn python, but I'm not sure what for yet. If I can think of a project that might be better to do in python, I'll use that to learn it.
Edit: just to add this as an important note, I do mean strictly for linux, not multi-platform.
Edit 2: I'm hoping for objective answers, like a specific project, not a general field of projects, etc.
My company was contracted to build a web application last year, and the client specified that it should be done in Flex. Now, this application should have been a web application, but we had a unique opportunity to try something new.
We had absolutely no idea what we were doing at the time, but it was a great learning experience. My advice would be to try something new when you get the chance, make mistakes, and continue to learn.
Might be harder if you want to learn Python casually... Try getting someone to pay you for using it.
From a coder's perspective, what kind of project should I choose python over php for where both could do the job?