Since no one has stepped up yet I’ll try to give an answer to this. It’s mostly gathered from reading http://css3pie.com/documentation/product-comparison/ and googling around. I only have hands on experience with CSS3PIE. If you know any of the other projects feel free to edit this
CSS3PIE by Jason Johnston
Adds good support for Css3 border-radius, gradients, box-shadow.
My experience with this is that it works pretty well and has great support for border-radius. Problematic for hover effects (I would expect all items on this list suffer from this) but seem flawless when used on static html.
More info at http://css3pie.com/
ie-css3.js by Keith Clark
Adds support for a lot CSS3 pseudo-class selectors. Is extremely small since it feasts on other js libraries you include, like jQuery.
This could work well with other projects on this list like CSS3PIE.
More info at http://www.keithclark.co.uk/labs/ie-css3/
UPDATE The ie-css.js project has been reborn here: http://selectivizr.com/
IE9.js by Dean Edwards
Works for IE6, 7 and 8. Make IE support transparent PNG, new CSS selectors like parent>child and :first-of-type, fixed positioning, max/min width/height support.
It's my impression a lot of work have gone into this over the years. Some reviews are really positive, some dismissive.
More info at http://code.google.com/p/ie7-js/
ecsstender by Aaron Gustafson
Works with IE6, 7 and 8. In a modular way adds support for CSS3 selectors, font-face, border-radius, box-shadow, colors and transforms.
Very well documented. More info at http://ecsstender.org/extensions
DD_roundies by Drew Diller
Adds some support for border-radius and png.
More info at http://www.dillerdesign.com/experiment/DD_roundies/
border-radius.htc by Remiz Rahnas
Very simple support for border-radius.
More info at http://code.google.com/p/curved-corner/
ie-css3.htc by Nick Fetchak
Some support for border-radius and text-shadow. CSS3PIE claim to do border-radius better.
More info at http://www.fetchak.com/ie-css3/