THE MOST ESSENTIAL are of course the standard general purpose foot and the straight stitch foot. Not counting putting in a zipper, you can really do lots with just those two. Here is my short list of other feet I use for practically every sewing project:
The walking foot
Known also as the even-feed foot, this clunky looking contraption does exactly that - feeds both the upper and lower pieces of fabric at the same rate, thus greatly reducing (if not eliminating) pattern non-match-ups and the upper fabric shifting as you sew. Great for quilt piecing and for sewing multiple layers. Pfaff did a brilliant thing when they incorporated the even-feeding technology (IDT) into a standard feature in many of their sewing machine models.
The zipper foot
This little baby does so much more than just help us put zippers in. It doubles as a piping foot, as a sew-in-the-ditch foot and works fantastically when putting in a busk in corset-making. Because it swivels, it adjusts itself as you sew while butting it against the hardware on the other side of the fabric, resulting in a very close stitching line. Indispensable, in my opinion.
All right, not a foot per say but since it attaches to the presser foot shank, I felt it needed to be included :) Most modern sewing machines come with really good 1- or 4-step buttonhole function. My 40 year old Singer 328k comes with none, and my 20 year old Singer 9805 does not do a good job of it (but then again, maybe it's just my lack of skill :P). This piece of mechanical wonder comes with 19 templates to make 4 different types of buttonholes in a variety of sizes/lengths. Zig zags are always even with nicely rounded ends. It makes regular type buttonholes, key-hole type, bound buttonholes and eyelets! Next to my vintage sewing machine, this was truly one of my happier ebay finds. If you are thinking of getting one for your vintage machine, I highly recommend it! Tip: instead of the feed cover plate included in the box, use your machine's drop-feed function or feed cover plate. It can get very fiddly trying to fit the buttonholer plate to your machine.
This is not a comprehensive list by a long shot, but what else do we really need, eh? What are some of your favourite sewing machine attachments and accessories?