Optifine - Makes the game more effiecient at rendering to increase performance and allows it to be more customized. So for example, you can disable fog, customize render distance so its half way between short and normal, disable particles affects, remove void fog, enable mipmaps, customize animations where they can be turned on/off,change full/screen resolution, set specific things such as clouds to fast mode while letting leaves be rendered in fancy mode. From experience, my game has WAY less lag spikes, and runs much more smoothly.
Here is a link: http://www.minecraftforum.net/topic/249637-125-optifine-hd-a2-fps-boost-hd-textures-aa-af-and-more/
SignLagFix - Makes it so that a sign's text is not loaded until you are within a certain distance of it. Its really convenient in the sense that when you go to spawn, you are not going to have massive sign lag. From personal experience, it doubled my fps at spawn.
Here is a link to the forum post:http://www.minecraftforum.net/topic/843697-101-sign-lag-fix-major-fps-boost-for-some-people/page__st__20
Here is a link to the download:http://www.mbcraft.com/files/SignLagFix1.2.5.zip
(Posted download because it was updated by a user, not the original mod creator, since it has not been updated for a while.)