FancyUpload is a file-input replacement which features an unobtrusive, multiple-file selection menu and queued upload with an animated progress bar. It is easy to setup, is server independent, completely styleable via CSS and XHTML and uses MooTools to work in all modern browsers.

More details here.

Having problems with image uploads on Wordpress 2.5? Use Flexible Upload plugin instead hacking .htaccess files.

Flexible Upload is a plugin for Wordpress 2.0.x, 2.1.x, 2.2.x, 2.3.x and 2.5 to enhance the image upload functionality.

Main Features

• Resize pictures at upload to the size you specify
• Let you decide whether or not to create a thumbnail and specify its size
• Optionally incrust a watermark signature in all pictures you upload
• Specify picture alignment (left, right, center) when inserting them into your posts
• Specify a plugin like Lightbox (Thickbox, Greybox) to display your pictures, or open them in a separate window
• Insert a caption for each picture
• Multi-language support