ONLY $20! Reg. $39.99. We have this exact one and it's a blast. The kids absolutely love it! Great idea for Christmas present!
71.5" x 40" Jump o Lene Jump n Gym [amazon.com]

my review: We've had this for over a year now, and it gets heavy use with 6 kids (ages 2-6). THEY LOVE IT! And so do us parents. It was easy to set up, just used a little air compressor to blow it up. It's pretty nice sized, and although it recommends only 2 kids at a time, we've had 4 in there holding hands and jumping and having a blast. The little door to crawl in and out of is perfect. Although I've seen other reviews regarding holes, ours has been super durable. (it does come with a patch if you need it) We use it in the yard or on carpet, not on concrete or anything abrasive. Highly recommend. Oh, and btw, we've used it indoors for parties and the kids love it there, too. Very very fun toy.

We also bought a bag of 100 balls to throw in there and let the kids roll in, but we haven't even done that yet because they have so much fun just being in there jumping, bouncing, rolling, etc. And they love to put it upside down and use it as a fort!

I got mine on a Black Friday deal 2 years ago for $28 (reg $40-56), but have been watching the price all this time so we could pick up another one (so we could have two going at the same time). This is the lowest I've found it for.

ADDED PHOTO FROM WHERE WE STORE IT IN OUR MESSY GARAGE.

Thanks to Lauranc99 for finding this Sesame Street ballpit [amazon.com]as well. I can't vouch for this one because I only have the Jump-O-Lene, but please see the pictures submitted by another mom on Amazon...with the kids in it, it is quite small. Still nice for a little one, and half-price at only $14.99 right now. Reps to Lauranc99, see msg #7 below, for this one.

Jump to this post to rep wadafak for finding an Intex 110-120 Volt AC Quick-Fill Electric Pump for $7.66 [amazon.com], also at Amazon. This is the exact one that I bought for $10, and we've had great luck with it. It has various size nozzles to use, and of course it inflates AND deflates. A must-have for these big inflatables. Thanks, wadafak.