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 is a contraction of です, but you will also hear (mostly younger guys) putting it (without the っ) on greetings. こんにちはす！こんばんはす！[Here's](http://ameblo.jp/tee-ameba/entry-10766309960.html) a real example (written like it's spoken).
っす Is *not* normal polite Japanese. Think of it as almost using a です when the situation is uncertain; for example, a group of young guys who've met fairly recently. です・ます are rather stilted, but they don't know each other well enough to use complete casual style. The contraction っす is a nice halfway point. 
In the context that you saw it, it could be expanded to これを使っているのです. Then then の turns into ん and then disappears. The "のです" construction is used a lot more often than regular old ます. Technically the first one is considered an explanation of something, but it is also less stiff than the latter.