Different people like different feet for piecing, and it depends somewhat on your machine. I just use a regular foot, adjust my needle to the right, and apply a strip of mole foam (a thick cushioning adhesive used on foot blisters, available in the foot section of a pharmacy) to the base of the machine as a guide to keep my fabric a scant 1/4-inch from the needle.
The walking foot has a lot of uses. Some people use it for piecing because it moves both layers of fabric together. Mine is really too wide to get accurate 1/4-inch seams. However, I like the walking foot for quilting straight lines and soft curves. I *always* use a walking foot when applying binding.
Here is a picture of the walking foot for my Bernina:
http://farm4.static.flickr.com/3172/...1daef81a79.jpg
Usually the walking foot designed specifically for your machine is the best. You can also buy a generic walking foot; sometimes those work well, sometimes they don't.
I use an open toe foot when I do machine applique (as described by Harriet Hargrave in her books). It lets me see the edge of the applique and where my needle is going in very clearly. Here is a picture of a Singer open toe foot:
http://thesewingcenter.com/images/386023050-P.jpg
As you can see, there is no bar to interfere with vision.
For free motion quilting, I use a darning foot because it "hops" with each stitch (allowing me to move the quilt). Here is a picture:
http://www.sewingworld.com.au/images...ning-foott.jpg
These also come with open fronts and/or larger circles, but I have found that the darning foot works best for me.