Here are some of my favourite modern sewing accessories. They're absolutely marvellous!
Janome Rotary Even Feed Foot
Top of the list. This wonderful little contraption means that you can sew thin fabric on your sewing machine, even if you don't have adjustable presser foot pressure! It makes all the difference in the world. You see, if you can't increase the presser foot pressure, the fabric swivels around when the needle is down, and goes to fast when you try to keep it taught, resulting in stitches worse than my hand sewing. (Try with paper for proof.)
But with this little wonder, you make up the height difference, and the rubbery tracts on the foot keep the fabric where it ought to be.
Vanishing Ink Pen
I like this for use with my rulers because it doesn't mark them and mess up my fingers like pencils and regular felt-tips pens do. It does vanish off the paper but you can simply draw over your vanishing ink with a water erasable pen.
This pen is also great for marking new lines on your toiles when fitting and designing. When you have finalised your designs your can transfer them or mark them permanently with a water erasable pen.
Rotary Cutting Tools
These are wonderful for pattern work. You can draw and cut perfectly straight lines at perfect angles so they are great for length alterations to patterns.
I like to use a fairly small mat (18"x12") because it is easier to move around to put away than the larger ones.
These rotary cutting rulers are much, much better than a regular ruler because they are thick enough not to be cut with the rotary cutter. (My regular rulers got chipped.)
So there are my top 3 Modern Sewing Accessories (not counting a sewing machine and steam iron of course). There was another one, but I can't remember what it was now. Oh well.