F-Spot - Lens Adjust
Many digital cameras have shoddy lensens which are fish-eyed, have abberations, and just look bad. Even the highest-end cameras have perspective issues sometimes. F-Spot should have a sidebar for adjusting lens values.
Chromatic aberration control (R/C & B/Y)
Vignetting (amount & midpoint)
Real-time lens transformations (correcting barrel distortion / fisheye effect / remapping to rectilinear, etc.)
Shift-lens emulation
Perspective transformations