This mod adds support for HD textures and a lot of options for better looks and performance. Doubling the FPS is common.

FPS boost (examples)

doubling the FPS is common
decreases lag spikes and smooths gameplay

Support for HD Textures

HD textures and HD fonts (MCPatcher not needed)
custom terrain and item textures
animated terrain and item textures
custom HD Font character widths
custom colors
custom block color palettes
custom lighting
unlimited texture size

See more content and download at: http://minecraftblogs.com/optifine-hd/