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, 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.
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.