I'm a swap hostess and I have to say Haveplenty hit the nail in the head for me... BUT in all actuality I love everything so for me you can never go wrong no matter what is sent to me... I do make quilts for charity so currently charm packs are my favorite...
A few things off the top of my head that I always need or use is:
Needles
Thread
Seam ripper
Tiny scissors
Nippers
Post it notes
Fine sharpies
ANYTHING handmade
Charm packs/ fqs / yards of my favorite fabs (1930/CW/Batiks)
Postage stamps
I don't expect anything from my swappers but it's a nice surprise when rec'd ...