If your looking for an alternative to the Flixel framework for developing 2D Flash titles, Chevy Ray Johnston (Beacon, Skullpogo) has released FlashPunk, his own ActionScript library designed to offer "a fast, clean framework for prototyping and developing games". He's not only put the tool online for free, he's posted some online documentation, written a beginner's tutorial, and setup forums.

FlashPunk is specifically geared toward creators who like to work with with 2D raster/bitmap graphics, as opposed to vector graphics. "It can manage thousands of animated bitmap sprites on-screen at a time without slowing, a lot faster than Flash normally could, because it operates under the assumption that your game primarily uses bitmapped graphics," explains the developer.

As for why you'd want to use FlashPunk over Flixel, Johnston has also shared technical details on the differences and even weaknesses of his framework so coders can decide which tool to use based on the merits of each. There are at least two projects underway using FlashPunk: Johnston's own Fight! MSD (in collaboration with Paul "Pietpiet" Veer) and Matt Thorson's Jumper Four.