It might not have much competition yet, but the most entertaining Flash game of 2008 so far is the inventive puzzler/platformer Cursor*10. At first, the gameplay looks extremely basic. Simply click on the "up" staircases to make your way through 16 levels that contain only stairs, pyramids, and buttons. Exploding pyramids give you 1 point apiece, whereas reaching the 16th level provides 100 points.
I won't ruin the game by explaining how to play, but the basic innovation of Cursor*10 is that you'll most likely need to use each of your 10 allotted cursors wisely and efficiently in order to even reach the finish, let alone collect pyramids along the way. Each of your cursors exists within the same time frame, so that by the time you reach your seventh or eighth cursor, you'll see numerous arrows scattering about the levels, blasting pyramids, pressing buttons, and scampering up stairs.
Once you've sent a few cursors through the paces, you'll figure out what's up and what you need to do to finish the levels. Then, you'll need to get a lot faster. I don't recommend playing with a laptop trackpad, though I have beaten the game using one.
After you've figured out the tricks and improved your efficiency to blast pyramids along your way to the top, you may find yourself slightly addicted. Have no fear; there is a cure. With a little practice, it's quite possible to collect all the pyramids and reach the final level in time, giving you a final score (the maximum, I believe) of 188. If I can do it, anyone (with a mouse) can do it!