I prefer latin names because the latin language speaks so much through one word.
Spolio: To strip, plunder, rob, despoil, loot. Perfect for a rogue type guild.
(Taken) Invado: To seize, attack, usurp. My favorite, it pretty much means to dominate everything.
Lacertosus: Strong, healthy, powerful. My name.. There is probably a type of way to say this as a meaning for a whole.
Syndicate: Not latin, but it is a pretty cool name for a group.
Sanguine Familia: Blood family, united in blood. Vampires anyone?
Illicitus: Forbidden. Pretty cool vip sect.
Those are some guild names I can think of off the top of my head.