SWFUpload is a brilliant tool built with Flash and javascript that allows you to upload multiple files at the same time, allows you to display progress bars and various other useful things. The front end is left wide open by the SWFUpload API so it gives you complete control over how the UI works.
Adobe Flex is a development platform for rich internet applications (RIA), based on the Adobe Flash platform. Flex projects, (like flash) compile into the .swf format, and therefore boast cross-browser compatibility (assuming the user has the flash player installed). Flex comes in two forms, for experienced developers, the open source sdk (software development kit), and a commercial IDE (integrated development environment) built on Eclipse. Here I will focus on the IDE, Flex Builder. In this article I will very briefly go over the basics of what Flex is and how to put a Flex application together.