ffdshow: Rapidly making codec packs obsolete. Plays almost everything.
Haali Media filter: Supports the Matroska, MP4, and OGM A/V container formats.
VSFilter: Supports a wide variety of subtitle formats.
DivX ;-) Audio: Just in case.
AC3 XForm filter: Makes life a little easier for folks with external Dolby Digital decoders.
HDTVPump: Support for HD/ATSC transport streams.