/particle command

particle <name> <x> <y> <z> <xd> <yd> <zd> <speed> [count] [mode]

► Create the Elder Guardian screen overlay

/particle mobappearance ~ ~2 ~ 0 0 0 1 1

► Create a love heart 4 blocks above whereever the command is executed.

/particle heart ~ ~4 ~ 0 1 0 1 3 force

► Create spell particles at a specific location.

/particle mobSpell -243 67 246 1 1 1 1 5

► Create the enhantment table effect and play it 1000 times.

/particle enchantmenttable ~ ~2 ~ 0 0 0 1 1000

Combine with the execute command for additional target controls.

► Create portal particles above all horses only if the horse is 10 or more blocks away from where the command is executed.

/execute @e[type=EntityHorse,rm=10] ~ ~ ~ particle portal ~ ~4 ~ 1 1 1 1 3 force

► Create a small thunder storm cloud particle around all Bats.

/execute @e[type=Bat] ~ ~ ~ particle angryVillager ~ ~ ~ 1 1 1 1 1

Notes:
Currently the only [mode] option is 'forced' which will force the particle to be seen by all players regardless of their particle effects video settings.

More information:
http://minecraft.gamepedia.com/Particle
http://minecraft.gamepedia.com/Commands