Bukkit

Like ChopTree but it has less lag server and client side :P

Suppresses TNT scattering and has a chance to reduce lag

Adds the /pingtest command useful for testing server lag.

Lagtest

LagEssentials is a plugin to help reduce server lag.

Unstick players who are glitching into the ground due to lag upon connection.

Caps those laggy mob-grinders by limiting the number of mobs that can spawn in a region over time

Changes 'lag' to 'fun' when posted in chat.

Limit the amount of mobs by type within a certain radius to prevent server lag

Lag-free world resets, anyone? If resetting is enabled, WR reverts you world folders from backups in a matter of seconds

Shows players their exact ping, and if set to, can kick players with a ping that is too high. Good for PVP servers.

This will reduce the basic lag of any server!

[] Lag Decreaser []

The ultimate PvP plugin, no lag and easy to use!

Anti Drop does not allow anyone to drop items.

LagChecker For Your Server.

Tool for culling zombies to reduce lag, while getting "pet" zombies with names

Hide Other Players To reduce Lag!

Lag client side, this plugin will hide players and disable in and out coming chat on command...

LaglessWeather

LagFixer will increase your server performance! Lightweight and efficient at the same time!

Limit Pointless Mob Spawns + Reduce Lag!

Possibly the greatest anti-lag plugin there is!

Cancel domage or death, due to a lag or regen, when a player log on or is teleported.

MercilessNoRain is an extremly lightweight plugin that removes rain which can cause lag.