Well I have a few higher priced games, but nothing crazy expensive. As for just rare, I mostly have oddball demo or promotional discs that would probably be considered pretty rare. For instance, I'm the only member on this site with the Muramasa: The Demon Blade Demo Disc for the Wii listed in their collection, although there seems to be two listed on ebay right now so maybe it isn't that rare after all.
Some of my favorite parts of my collection aren't necessarily rare individually, but possibly rare to see a complete collection of. For example, I have the every Official U. S. Playstation magazine demo disc, Official Xbox magazine demo disc, Game Wave game, and Xavix Port game.