I occasionally find myself using various URL parameters that Livejournal supports. To my knowledge there is no single place that documents them all (no, I did not search the web to find out), so here's what disparate information I collected. I may amend this post occasionally, so feel free to bookmark it; and if you know of anything that's not covered here, by all means let me know.
- Keeps Livejournal from applying user styles and uses the default site scheme instead: specifically, the one that you seleccted, or Horizon for logged-out users. Documented in FAQ #259.
- Displays entries as raw (escaped) HTML. Documented in FAQ #259.
- Displays content in a certain S2 journal style. You can go to Your Styles for ID numbers of styles you've created/customized yourself; other than that I don't know how IDs relate to styles. Mentioned in FAQ #177.
- Used on your friends page, determines whether communities (C), news (N), personal journals (P) and/or syndicated accounts (Y) will be shown; any combination is possible. No effect on other pages. Documented in FAQ #259.
- Displays content in your own journal style, rather than style of the journal you're viewing. Useful for styles that make entries difficult to read, and often more visually pleasant than ?format=light. Documented in FAQ #259.
- Displays site pages in the specified site scheme. Documented in FAQ #186.
- On My Account Settings — Display, show XColibur and Dystopia. No effect on other pages.
These are all the ones I know so far.