Don't expect a lifelike simulation of a 3D snowglobe and you'll be happy with this app. The snowflakes move fairly realistically (except for the initial scattering), but it all takes place on a 2D plane, so much of the fun in getting a real snowglobe to swirl its flakes like a blizzard is missing, but the app does let you put your own picture in the middle, and it tracks the orientation of the iPhone so you can make the flakes fall in any direction. Snowglobe also makes a nice demo for accelerator functionality.
Review update: A really cool modification would be the ability to change the 'flakes' to other shapes depending on the season or your mood - orange pumpkins, brown turkeys, red hearts, etc. The demo on their web site convinced me to buy this - I put the Joker in the snow globe! Plus, it saves your last configuration. Fun!
Download some holiday fun! Turn your phone into a classic toy with snowflakes that swirl around, falling whichever way you tilt the phone. Touch and move your finger to add more flakes to the globe, and shake the phone to really get things flying! But don't shake too much, or you may get a surprise :) Choose from wonderful built-in scenes, including a Christmas tree with flickering lights, a snowman, baby polar bear, dancing penguin, and a bunny giving out carrots as holiday presents. Or take a picture with your iPhone camera or choose one from your photo library to show inside the snow globe. Fully customizable with adjustable snowflake size and speed, optional sleigh bell sound effects when you shake the phone, and the ability to make your own scenes. Or if you aren't feeling wintry, turn the snowflakes into rainbow colored confetti. - Now with the ability to show live video inside the snowglobe!