Most slot machine game downloads are completely free. The online casinos are so eager to have you as a player that they'll let you try their software for free, and usually they'll offer you bonus money to play for real money. With high speed connections becoming more common, it's becoming easier and faster to download slots.
Realtime Gaming is our favorite slot machine casino for downloads. Their progressive games like It's Good to Be Bad and Mid-Life Crisis are unique, fun, and play really fast. A lot of Realtime Gaming casinos have great offers for new players too - we recommend Win Palace Casino for their excellent customer service, and they have both an instant play and downloadable casino available.
Playtech slots casinos only have around 40 slot machines each, but they have some of the most well known names and have a very nice variety. Europa Casino is our favorite Playtech casino, but Playtech no longer accepts US-based players.
Microgaming offers over 100 different slot machine games with their downloadable software, almost all of which are attractive and unique. Microgaming casinos do not accept players from all 50 US states though, but they do take players from 39 states. RoxyPalace is our recommended Microgaming download.
Cryptologic, also called Wagerlogic, offers some awesome slots, including the Marvel comics slot machine games, but again, they don't accept US-based players either. If you're not from the USA, and you're interested in Cryptologic casinos, check out Sands of the Caribbean casino.