tpaksu

This plugin uses CSS technique to fix the png image transparency issue and jQuery to apply this script to all of png images shown in the page. 
This doesn’t work for positioned and tiled png background images. Maybe in next versions.

WP pngfix

Plugin that fixes the png image transparency issue on IE.

Diego Betto

Simple plugin that renders a checkerboard under PNG and GIF images so you can see alpha channel

Media Checkerboard

Simple backend plugin that renders a checkerboard under PNG and GIF images so you can see alpha channel

WP pngfix vs Media Checkerboard