Modder 'IcE La Glace' has been making steady updates to his iCEnhancer mod for Grand Theft Auto IV, which essentially enhances the game's graphics to be all they can be.
One of the latest trailers shows off what it can do best; check below to see the variety of added advanced effects in action, including cloudy and rainy weather, improved textures, better lighting, depth of field, and others. At its best, it's not far from being photorealistic, and this is just at 720p.
Note the footage is jerky due to the intensive nature of recording; actual gameplay should be smooth provided you have the gear to support it properly (one YouTube user jokes, "Computer specs required: GlaDoS", but IcE says a 1GB GPU is fine).
The newer 1.2.5 beta version of the mod adds even more effects including FXAA and several different options for depth of field, among other items. If you're interested, hit the source to download. Surprisingly, it's a mere 2MB file. Update: 1.2.5 Steam-compatible version is here.