http://curse-gaming.com/mod.php?addid=111
Had this mentioned to me by a fellow rogue the other night and it's become an essential part of my toolkit now
Basically it allows you to define "sets" of weapons for main and off-hands and toggle between them with a single keypress.
Where this is useful for rogues is if they want to use a dagger for an ambush, then switch to a more powerful weapon for full-on combat.
I have my dagger with crippling poison applied in my main hand, and a decent damage 1-hand sword with instant poison in the other - this is set 1 and used when stealthing so I can ambush or cheapshot monsters, hopefully landing the slowing poison.
Then a quick keypress and my off-hand sword moves to the main hand and a secondary sword goes into my off-hand, with deadly (DoT) poison applied.
I didn't follow the install instructions directly as I use Cosmos already - so I unpacked the WeaponSets folder into Interface\Addons rather than directly under Interface.
There are many more WoW addons at the main site too, not used any of the others though:
http://curse-gaming.com/