In addition to the recently leaked ZFS Read/Write beta, AppleInsider has found hidden Leopard features.
Quicktime player has reportedly been updated to include support for full-screen movie playback without the need for a QuickTime Pro key.
DVD Player is also reported to be set for Blu-Ray and HD DVD disk support as well as the opportunity to add DVD cover art and change movie region support on the fly. The site had previously reported that DVD Player would a fair amount of attention in Leopard. The site does not make it clear, however, whether this support is in the current build, or is rumored for future builds.
Leopard's version of iChat now supports Google's GTalk (a report confirmed by MacRumors sources as well). If more protocol support is desired, Apple has built in the ability to set the default IM client so users can use a 3rd party program (such as Adium). AppleInsider reports that this configurability extends even into Dictionary.app, which can now use either the built-in Oxford dictionary or Wikipedia.