Boxee will do exactly what you want. Nice and small too. It will pull over the network and supports just about every format:
Supported formats/codecs:
Physical media: CDs, DVDs, Video CDs (including DVD-Video, VCD/SVCD and Audio-CD/CDDA)
Container formats: AVI, MPEG, WMV, ASF, FLV, MKV, MOV, MP4, M4A, AAC, NUT, Ogg, OGM, RealMedia RAM/RM/RV/RA/RMVB, 3gp, VIVO, PVA, NUV, NSV, NSA, FLI, FLC, and DVR-MS (beta support)
Video codecs: MPEG-1, MPEG-2, MPEG-4 (SP and ASP, including DivX, XviD, 3ivx, DV, H.263), MPEG-4 AVC (H.264, including Nero Digital), HuffYUV, Indeo, MJPEG, RealVideo, QuickTime, Sorenson, WMV, Cinepak,
Audio codecs: AIFF, WAV/WAVE, MP2, MP3, AAC, AACplus, AC3, DTS, ALAC, AMR, FLAC, Monkey's Audio (APE), RealAudio, SHN, WavPack, MPC/Musepack/Mpeg+, Speex, Vorbis and WMA.
Digital picture/image formats: BMP, JPEG, GIF, PNG, TIFF, MNG, ICO, PCX and Targa/TGA
Subtitle formats: AQTitle, ASS/SSA, CC, JACOsub, MicroDVD, MPsub, OGM, PJS, RT, SMI, SRT, SUB, VOBsub, VPlayer