Cornice is a cross-platform image viewer written in Python + wxPython + PIL. It doesn't pretend to be complete, fast, or even useful, but I like it and it is the viewer I use on both Linux and Windows. It has been inspired by the famous Windows-only ACDSee.
Why did I write it? Well, because I like ACDSee, but it's not free and it doesn't run on Linux, which is my main platform. There already exists an ACDsee-like viewer, GTKSee, but it is unmaintained and it lacks some features I wanted (bookmarks, a good keyboard navigation and zooming). First I tried to add such features to it, but then I decided to rewrite it from scratch, so that I could use it also on windows (and also because I had some troubles, especially when trying to port GTKSee to the gdk_pixbuf lib, and also because Python is more fun than C.