HD Texture Packs
Estimated Cost: $0
If you're running a crazy-fast computer with loads of RAM, your concern likely isn't optimizing frame rates. Modern developers do all they can to curb minimum PC specs, and part of this process is reducing the overall size of their textures.
Leave it to the enthusiast modding community to unlock maximum potential by creating new texture packs that replace the standard, truncated textures with shiny new high-resolution replacements. These packs will make your game look fantastic and much more details.
Do note though, that these are only for players with very high-end machines. Putting one of these in a low-spec machine is generally considered suicide.