I use a HTPC with TheaterTek.
Works great. Custom aspect ratios are simple and fast to implement. It can do an "auto-detect" on aspect ratios (haven't really bothered to try), or it remembers your settings for each individual title. There is a free trial download last time I checked, give it a whirl.
TheaterTek will play back DVD/.vob and .ts files. For Divx video, Media Player Classic (freebie) will do the necessary stretching. I haven't done an a/b comparison, but MPC has pretty good PQ.