I am generally a big illustrator/photoshop user, but I happened to make all the banners in Fireworks. The effect is usually called a gel effect or glass, but I tend to make it simpler without inner shadows or shading.
Here I use a solid background and overlay a white ellipse (others I use a rounded rectangle) with the transparency set between 10 and 26, depending on the background color. Really simple, but a pretty clean effect.
For the ones like this:
I took a square, used the circular seclection tool and subtracted it, creating the arc, and set the transparency.
Hope this helps, and glad you like the banners. They aren't super snazzy, but that's not what I was going for anyway.