There are various shaderpacks available for Minecraft. This page covers popular ones and helpful options to use on them. Since shaders use the GPU heavily, don't use shaders (or use minimal setttings) if you have a low end computer.

Loaders

Loaders configure graphics settings and choose the shaderpack of interest. Loaders are client-side mods and can be used on servers. When opening the loader, click Open Shaderpack Directory to locate the directory to place shaderpacks.

Shadermod

Shadermod is the original shader loader.

Optifine

Optifine has incorporated Shadermod into itself and can load shaders.

There are several things to be watch out for when using Optifine

Optifine and the Fastcraft included with GTNH conflict. Remove Fastcraft from your mods directory, or change the extension from .jar to .jar-donotuse
Dynamic lighting works well with GTNH, however you want to make sure to use the Fast setting. Any higher causes severe lag because of constant lighting updates. You can put torches and other light sources in your player's right hand in the Mount and Blade slots and use torches that way with a weapon.
Most of the other settings work fine.
F7 to show mob spawning spots and F9 to show chunk boundaries do not work with shaders.

Shaders

Shaders are scripts written to use OpenGL commands to add extra graphical effects to the game.

SEUS 11.0

This shader will require some manual changes in order to be useful for you. First extract the ZIP file into the shaderpack directory under SEUS 11.0

Open composite1.vsh to change torchlight to pure white
Open final.fsh to change water fog to 0.05
With SEUS 11, you will have issues with nightvision. When using nightvision you will need to switch shaders.

Sildurs Vibrant Shaders v1.2

This shader supports most configuration options within Minecraft under Video Settings -> Shaders -> Shader Options

Emissive Light setting allows changing torch color
change moonlight brightness to Very Dark for proper atmosphere
SVS v1.2 will not properly display the moon. Disable the shader to see the moon phase for launching to the moon

Interior shot using Sildurs Vibrant Shader v 1.2 Extreme