Sony PlayStation 3 is still the best piece of home cinema equipment you can currently buy in my opinion.
It will play all of the formats you mention (including MP4 or H264 within WMV containers) and it will also play DVDs and Blu Ray discs. If you live in North America you will have access to an online video store where you can buy episodes of TV shows and movies just like iTunes.
You can also buy a TV tuner for it (if you live in Europe or Australasia or anywhere with DVB-T signals) which will allow you to record over the air broadcasts and pause and rewind live TV just like Sky+, V+ or Tivo.
You can copy files directly to the console's hard drive, which itself can be upgraded as it's just a standard 2.5" laptop SATA drive. I have 320GB installed in mine. It has a web browser which is compatible with YouTube, BBC iPlayer etc.
Oh, did I mention it just happens to be an extremely powerful games console with hundreds of games on the market?
Yeah. You could spend your money on AppleTV, but I don't think that would be good value at all when you consider what PS3 can do.