Note: For the sake of simplicity this tut uses a selection, for better AA on the edges, etc. use an alpha mask.
Plugins needed: N/A
Step 1. Open a new canvas or an existing image. For this tutorial I'll use a blank 800x600 image. Add a subtle gradient to the background layer so you can better see the highlights and shadows on the ball, then click the
button.
Step 2. Choose the
, and hold shift to make a circle. Select the 
with a fairly large size, and black set as your primary color, then brush where you want your shadow to be. In this case, the top left corner. Do the same in the opposite corner with a white brush.
Step 3. Next gaussian blur at a radius around 50, but this will change depending on how large your circle is. Now lower the opacity to 150-200.
Step 4. Now click the
button, and make sure you still have your circle selected. Now brush where your shadow is, but with a smaller brush size than last time. Do the same with the highlight.
Step 5. Gaussian blur again, this time around 25 radius. Lower the opacity to 125-175. These 2 steps define the shadow and highlights. Make sure you still have the original selection.
Step 6. Now add another layer,
, and select the again, with a brush size about 70-80 pixels. Set white as your primary color and click once in the middle of your glass ball. Gaussian blur at a radius about the same as the brush size you set. lower the opacity to around 150.
Step 7. Now for the specular highlight. Add a new layer,
. You should still have the original selection. Now lower your brush size to the 30's, and click once with white as your primary color. Lower the brush size by 10, and click again to below and to the left of the first click. Lower your brush size by 5, and click again with about half the 3rd dot over the 2nd one. 
Step 8. Gaussian blur this new layer at 10-15 pixels. Add a new layer, and set black as your primary color. Select the secondary color, click the "more" button and moce the "transparency-alpha" slider to 0. Using the gradient tool in
, click in the middle of the circle and drag to a little outside the circle. If the black is on the inside, right click on one of the nubs to reverse them.
Step 9. Use the
to resize the shadow. Make sure the bottom of the shadow lines up with the bottom of the circle. Lower the opacity depending on how dark your background is. Now that we are done with the selection of the circle, we can move the specular highlight back where it was. go to that layer, select all with ctrl + A, then move it back.
Step 10. Now you probably noticed that there are some random black pixels on the outside of the circle. Turn visibility on and off to figure out which layer it's on. Once you have, you'll have to manually go in and erase the black pixels. I'm not sure why this happens, but if you use an alpha mask they don't appear.
Your Done! You just made a glass ball. Play around with the level of blur and layer opacity of your ball to suit your preferences.
.
