Does the particle "を" (wo) have a special use when at the end of a sentence? I thought the character "" (wo) was only used for the particle whose only job was to indicate the direct object of a verb. But today I saw it at the end of an exclamation on a sign I think on a shop: > !! So what job is doing here?

It's still the object marker. The sentence is just not finished and the verb is implied.

(there was a question mentioning suspended sentences but I cannot find it for the moment)

Anyway it's often used:

> {}
>
> May the force be with you!

Additionally it gives some kind of propaganda feeling to the sentence.