You have Wagerworks (one of my preferred software) but only on their last slot Wheel Of Fortune Triple Action Frenzy (which is just brilliant).
You can play it for free at BlueSquare:
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There is also B3W but only with 5 of their slots (they have the same engine, the only difference is the theme and the denomination).
My preference (just because of the theme) goes to Las Vegas Show.
You can play it for free at Casino Solera:
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I also remember that the Jackpot 6,000 slot from Netent has a gamble feature.
You can play it for free at CasinoEuro:
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MicroGaming has also 9 slots with a gamble feature but it's the UK pub slots.
You can play them for free at LadBrokes:
The 2 new ones are very funny: Pandamonium and Dubya Money.
Just wanted to mention that Player Preferred is the software which runs the HypeGaming website (you have to download).
EDIT:
You also have a lot of Gambling Federation slots with the gambling feature...but I wouldn't play there if I was you