OptiFine adds support for HD textures and a lot of options for better performance.
OptiFine is Completely FREE!
It allows Minecraft to run faster and look better with full support for HD textures and many configuration options.
“This mod optimizes your minecraft game world and increases the FPS. And of course, the gameplay will become much better and more interesting."
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OptiFine HD is a performance related plugin that can provide a large FPS boost and dramatically reduce lag. OptiFine also contains many advanced configuration options available from the video settings menu to further tweak performance.
Configurable Smooth Lighting
From 1% - smooth lighting without shadows to 100% - smooth lighting with full shadows.
Connects textures for glass, glass panes, sandstone and bookshelf blocks which are next to each other.
Removes the gridlike pattern created by repeating blocks of the same type. Uses rotated and flipped variants of the base block texture.