First up is the Blast Zone Hydro Rush Inflatable Water Park. It's got a water slide, a splash pool, a water cannon, and a climbing wall. All you have to do is inflate it using the included air pump (it's supposed to set up "in minutes"), hook it up to a standard garden hose, and your kids are all set for hours of water fun. You'll want to stake it down (stakes included) to make sure there aren't any mishaps, of course. It has room for up to six kids to play and splash their way through the day, although the maximum weight on the inflated structure is 200 pounds. Only one child at a time should use the slide or the climbing wall, and up to four kids can fit at once into the splash pool. It is recommended for ages three and up, and will cost you about $400.00 US.
Source: 7 Gadgets
Then we have the Wham-O Monorail Water Slide, for a slightly smaller and considerably cheaper option. If you remember Slip 'n Slide from when you were a kid, this is the new and improved version. Instead of sliding down it on your belly or back, this has a giant sled for sliding down its 16-foot length which is kept straight with a central rail. It also doesn't just come to an end, it's got inflatable bumpers to stop you from overshooting the slide. That should save quite a few grass burns (I'm not the only one who constantly either slid off the side or the end, am I?). The bumper also makes a wall of water, and the central rail is where the water sprays from to keep it slippery. It comes with two anchors to keep it in place, and two repair patches in case of an accidental puncture. This one only costs about $20.00 US.
Source: Craziest Gadgets