General Sewing supplies:
machine in good working order, foot pedal & plug (yes, people forget these!) - if you need to and have time, take it in for a tune-up before the retreat!
extra machine needles
any machine accessories you need (walking foot, FMQ foot, extension table, attached light, etc.)
depending on the setting, a small light for your area (not all rooms or all spots in the room have great lighting)
good scissors - large for cutting fabric, small for trimming threads
rotary cutter & extra blades
most useful rulers - usually a 6 1/2" x 24 & a square up ruler (6 1/2" or 9 1/2" or maybe 12", depending on what size blocks you plan to use) .. remember any specialty rulers you need for projects
cutting mat - 18" x 24" is good size to have at your table, usually there are a few large mats provided or shared by others at cutting stations
iron & ironing board (maybe - some places provide them or only permit a few limited ironing stations)
seam ripper, pins, pin cushion, hand sewing needles, safety pins
marking pens, permanent fabric pen, pencil, scratch paper (graph paper works great for last minute designs & quick sketches)
neutral thread for piecing - maybe a couple of light neutrals and a couple of darker neutrals
prewound bobbins w/ your neutral threads & empty bobbins for if you use a different thread
surge protector

If you are not participating in retreat projects, plan some projects to take. Pack the supplies for each project in its own ziplock bag. If possible, pre-cut any fabric. Make sure you include the fabric, the pattern, any templates that you need, thread (if you can't use the neutral), pre-wound bobbin to match, and any other notions that you need for the project. This way you can just pull out a ziplock bag and start working.

If you are participating in a retreat project, package the supplies for each project in the same manner.

most retreats have a snack table - so bring something yummy to share.

plastic cup to have a drink at the table (but be very careful w/ food & liquids around sewing areas!)

painters tape
duct/packing tape - to tape down any wires that might be in the way.

If you're working on your own projects, I've found it best to have the cutting done ahead of time. Cutting space is usually limited and not the right height. Clothes that can be layered -- hard to get everyone to agree on a "comfortable" temperatre - be flexable. The list that was linked to above is a good start.
One year a friend forgot her power cord. Snacks, wine

eye drops, Aleve, Aspercream, Tylanol PM, Water,

Have organizer keep emergency contact information and/or medical information.
Got this from a Fons & Porter Tips: quilter had a problem and nobody knew who to call.

All your sewing & quilting supplies with your name on them!!!!!! (Important!!!!)

chocolate enough to share
favorite supply of drinks
aspirin(you'll need it because of all the laughing you will do) LOL
comfy clothes
comfy shoes
cushion/s for your bottom & back
sense of humor
brag book of your family & quilts you may have done

Great ideas. I could of used a small fan at my last retreat. And I had forgotten my seam ripper.

The first one I went to I didn't take enough un finished projests to switch to when I got bored with one. I also learned this trick from a freind, she puts all her sewing stuff in a medium suitcase and just wheels it in. Unlike me I with fifty million tote bags and struggling to carry all of them.

So how many projects would you take next time?

The last one I went to I took a quilt I had just started and 2 that were partly dona and some scappy blocks that I didn't have to think much about. I could sew and talk with those ones. Too many is better then not enough to swap if you get tired of one your working on.

I can't open the link . . . :-(