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's a contraction of です. It's not quite as polite as that though - it's always sounded a bit like "thinking that one needs to be polite but not bothering to do it properly" to me. I guess it comes somewhere between teineigo-level polite and casual in the politeness spectrum.
It's lazy polite form. Dropped for ease of use and to add a level of casual feel. Used nationwide.

When I worked in bars and a few host clubs this style commonly used in place of normal 敬語 as it is too stiff for young women, who are the majority of our customers. However, we always reverted back to normal 敬語 when an older male, female(ママさん) or couple was the customer. 

I suppose you can also think of this as a slightly flirtatious polite form.