I've been tempted to get a fisheye lens (The Nikon 10.5 DX specifically), and have seen the Fisheye-Hemi plugin for "normalizing" fisheye images.
Does anyone have experience of using the plugin or know of free or better alternatives?
Fisheye-Hemi is superior to these, though, as instead of using a standard remapping to a rectilinear projection that softens the edges and requires an aggressive crop it uses a custom mapping algorithm that preserves much more of the original image and sharpness and still straightens out lines.
This blog post is interesting, the guy fed an image of a grid pattern through the Fisheye-Hemi plugin and then created a warp transform in Photoshop that approximately matches the deformation on the grid. Pretty clever.
I use Gimp and the MathMap plugin, together with a few scripts of mine to convert the fisheye image into whatever projection better fits the subject: either rectilinear, stereographic, or Mercator. Mercator is my favorite: it's a kind of panoramic cylindrical projection that looks quite similar to the Fisheye-Hemi projection and is free of local distortions (mathematically speaking, it's a conformal mapping of the sphere).
More recently, I've written a script to provide the Lambert conformal conic projection. This one is useful for pictures taken with the camera pointing slightly upwards or downwards, and when you want verticals to be rendered straight, yet preserving their convergence and thus the the upward/downward perspective. This projection, like Mercator, is free of local distortions (it's conformal). Actually, the Mercator projection is just a special case of Lambert's conformal conic.