Games Workshop makes three spectre models already but if you want to use more than three it can look like an army of undead clones. However it is easy to convert spectres from almost any model.
Uncommon in Middle-earth, Spectres are cold, white, incorporeal figures akin to Ghosts. Victims killed by Spectres frequently become Spectres themselves. A form of spectre was encountered by Frodo in the Dead Marshes, these were known as Corpse Candles. Corpse Candles can create the illusion of being a whole, healthy creature by cloaking the hideous remains of its once-living body. They lurk in water and exudes a beckoning light that magically attracts victims.
This spectre was made using a Gorbag model. His sword arm has been repositioned. Using some green stuff I modelled a stomach wound so that is insides are spilling out.
This spectre was made from a Man of Númenor from the Warriors of the Last Alliance set. This one has had it's shield replace from a plastic Warg Rider sprue. The shield has had bits cut out so that it is aged and warn.
As with all cloak wearing spectres I have added green stuff to the bottom of the cloaks to give the effect that they are old, worn and tatty. I have also created another Man of Númenor spectre keeping it's original shield.
This spectre has been made from a plastic Warrior of Rohan. It's shield has been replaced with one from a plastic Warg Rider sprue and again has been cut to look worn. His sword as been nicked and bent with pliers to age it. Using a sword again from the Warg Riders sprue (a very useful sprue for bitz), his head has been cut and the sword glued in place to represent it's death wound.
A spectre can simply be made from using a Warrior of Rohan and painting like your other spectres (of course this one has had a shield replacement).
This spectre was made from a Denethor model. A pin vice has been used on it's face to drill out the eyes making it look skeletal and mess it up a bit.
An axe (again from the Warg Riders sprue, see how useful that is) has been glued to his chest for the wound that caused this poor souls death.
This spectre has been made from a Gamling model. It's head has been removed and replaced with an old plastic Warhammer skeleton model I had laying around. Some hair sculpted from Green Stuff as attached. I have replaced it's sword with a thigh bone from the same skeleton model.
This spectre is made from a plastic High Elf from the Warriors of the Last Alliance set. It has had both arms cut off and it's sword cut to be a one handed bladed. It's sword arm were then repositioned on the body. I left one arm off as a wound.
With more and more models being released, such as the plastic Dwarves it could be a nice idea to make some Dwarven Spectres and maybe even a Hobbit spectre or two?