Features that stand out to me: PSD/animated GIF/transparent PNG support - with a user-selectable transparent background color. Also zoom/pan/mouse controls, as well as various window resizing and positioning options. The status bar can also be hidden.
Pictus is a fast, free image viewer. It is designed to be small, fast and just totally sweet. If you are content with your current photo viewer software you can still install Pictus and just let it help Windows Explorer show thumbnails for even more images.
Now with portable support added in v1.2.0. Comes in 32/64 bit versions, and is open source. Have yet to try the PSD thumbnail aspect of it as I tried the zipped version.
Default keyboard shortcuts:
- Image navigation = Home/End, PgUp/PgDown, Space/Backspace
- Rename current image = F2
- Random image = R
- Move image to recycle bin = Del
- Bypass the recycle bin and delete the image immediately = Shift+Del