Use of these mods is not required to play.
There are many awesome client mods for Minecraft that make our server much easier to play on.
Here's a few we recomend you use:
OptiFine (HD texture fix, more video options)
HD textures, HD fonts, BetterGrass, much faster Smooth Lighting, options Dripping Water/Lava, texturepack change, fixed bug in Far View
Rei's Minimap (Beautiful Minimap)
Fast processing, Waypoints (Can not teleport),
ZanMinimap frame appearance
Beautiful map rendering:
Expression of semitransparent blocks
Bump-map expression of height
Expression of dynamically changing lighting
Automatically generate the block colors from your texturepack