mckwilter

The opinions listed below are mine and no one else's, determined through trial and error during 15 years of quilting.

Supreme Slider - great for FMQ, but don't get one if you don't FMQ. If you do get one, get the biggest size they have. Or, as suggested by others, check out the kitchen supply sites and get teflon oven liners.
Polly Paper - I haven't used this, as I have a roll of freezer paper I purchased 10 years ago. If the freezer paper curls, cut off what you need and put weights on it to flatten it out.
Sew Steady Quilter's Wish Table - also great for FMQ, but for regular piecing isn't really necessary. Check out YouTube for a tutorial on making a comparable surface for FMQ.
Wrappable Light - I bought one and then found I didn't have anything to wrap it around, so it went into my guild donation box.
Karen Kay Buckley's Perfect Pins - I LOVE these. They are so fine and sharp, but the tiny heads make them hard to pick up. Also, they are VERY easy to bend.
Porta-Trace Light Box - if you are doing applique, a light box is an essential, but you can make your own for much less -- if you have a dining table with leaves, remove a leaf and place a piece of glass or plexiglass in the space and put a lamp under the table. Or get a clear or opaque plastic box, turn it upside down and put a lamp inside it (I use 4 LED flashlights that lay flat). If the box has bumps on the bottom, turn it right side up, place a piece of glass or plexiglass on top and the lights inside it. For a portable light box, I bought one of those 12x12x2 storage boxes with the handle, used double stick foam tape to take the lights on the bottom. That way, I can also carry tape, pencils, etc.
Goddess Sheet (to protect surfaces from fusibles) - parchment paper will do the same thing or there are other sheets that cost less.
Thangles - great for making HSTs. I have a package in every size from 3/4" (for minis) to 4" finished. However, there are lots of different templates, including Debbie Tucker, Quilt In A Day and June Tailor, for making HSTs. Or you can use the Triangulations, as someone suggested, or print them free.
Mighty Bright Sewing Machine Light - There are less expensive options out there, including LED strips that are secured to the bottom of the harp of the machine.

Some things that I think are absolutely necessary --

Serrated blade scissors. There are many brands, including Karen Kay Buckley Perfect Scissors (I have 2 pair of each size), Kai (I have 2 of the applique scissors), Havel and Gingher. The serrated blade pulls the fabric into the teeth, rather than pushing the fabric away the way smooth blades do. Get a pair in every size.

Karen Kay Buckely Perfect Circles (both sizes), Perfect Ovals and Perfect Stems if you do applique.

Clover seam ripper -- First choice is the one with the white handle; second choice is the one with the flat brown handle. To me, they have the finest, sharpest point for getting under threads. With all the other seam rippers I've used, the point was too thick and not sharp.

Spray starch -- make your own, Best Press, StaFlo. Spray starch is a must.

Good rotary cutters in all sizes -- 60, 45, 28 and 18. Each has its purpose.

June Tailor Shape Cut - if you don't have an Accuquilt Go and do a lot of strip and subcutting, this is the one thing you should have.