The Wiki Rule: In addition to probably having one for every single one of its settings, the core rulebooks for 3.5E plus the Expanded Psionics Handbook were ported to a website under the Open Game License, The Hypertext d20 SRD.
Deleted Scene: Found on the DVD, with commentary. Some were removed to stop plot-holes.
Though there's also the scene of Ridley and Marina in the map, which was removed despite containing the setup for the entire story. Some very awkward exposition was shoved in afterwards instead.
Executive Meddling, Screwed by the Lawyers: First-time filmmaker Courtney Solomon tells a harrowing tale of meddling by TSR's owner (who rejected several famous directors, including James Cameron and Francis Ford Coppola), by the threat of a lawsuit from Wizards of the Coast who were trying to reclaim the movie rights; the terms of the settlement required, among other things, that he use an old script that had been approved by TSR years ago instead of an updated version that he had wanted to use. Source.
Money, Dear Boy: Irons rather famously took the role of Profion to pay for the refurbishment of a castle he had just bought. How appropriate.
On the other hand he seems to be having a wonderful time playing an EVILLLL wizard. Certainly more fun than the audience is having.
What Could Have Been: There were a lot of things that had to be cut because they didn't have the money to pull them off. Some of these unfinished scenes are on the DVD. Also, see Executive Meddling above.
Note that the most expensive cut scene actually contained the entire set-up for the main plot! Solomon was then forced to shoehorn in some awkward expository dialogue in the scene directly afterward.
The Animated Series:
Executive Meddling: Not much, but still present. Eric turned out the way he did because of this.