Question: I saw a sentence like:

> 
> 「それ使{つか}ってる**っ**す。」 
> 
> 
> 

and none of my dictionaries have an entry for just "っす". Is it a verb form, gobi, or something else?

Answer: 
It is a contraction of です--though as you'll note in your example sentence, it is often used in places where です would not be grammatically correct. It indicates politeness, but shows the (often young) speaker's ignorance of the proper polite forms. :)
Similar to English, you can abbreviate common words with slang. I've heard おはよう become おっす, and similar. There isn't even a す in おはよう.