I'm in the process of generating tiles with generate_tiles.py and I'd like to write a Python app to display them instead of having a web interface. Does anything like that already exist or is there information somewhere on how to write such an app myself?
You can look/edit your map using a desktop application like Tilemill or QGIS (I can't really guess why are you generating tiles if it is not for a web application).
Tilestache has an internal webserver you can use for testing, it's easy to get it running just using config files.
A few pointers to get you started (or keep you busy for a few weeks):
Server side libraries and applications
Client side libraries
Tools for custom map creation
Markers
Some related videos from PyCon2012
Some interesting applications to get you inspired: