I saw a sentence like "それ使ってるっす" and none of my dictionaries have an entry for just "っす". Is it a verb form, gobi, or something else?
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 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.
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.
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. :)
As others have noted, it's a corrupted form of ーです. It's usually found in social situations where both humility and some roughness are apropos, like in yakuza movies, when a lower-level thug is talking to the boss. Another example is when addressing sempai in after-school clubs, particularly athletics. おはようございます -> おはようーっす！, お疲れさまです -> おつかれっす, etc.
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.