J.S.
In my last year of work before I became a SAHM, our department had a "babyboom". Every woman except 1 had a baby that year -- so we had lots of showers. We did "potluck" lunch in a conference room and all chipped in to buy some of the big items on the mom's registry. Most of the moms sent the group an email "thank you" (we were a group of software developers, so this was not considered in bad taste, but it might be for non-techy groups) and then brought in a food item for the group to share. Once the baby was born, the mom would post some pics on our "Baby on Board" website and usually write a little journal entry with a thank you. I would not expect an individual thank you unless I gave an individual gift or if I planned and organized the whole shower.
As far as what to write, following is my personal formula for thanks yous in all occassions:
Thank you for the [gift]
A statement of how appreciated the gift is
A statement of how I plan to use the gift, where I put the gift in my house, or something to that effect (i.e. "Thank you for the crystal bowl. I have filled it with floating candles to make a beautiful centerpeice" or "I was so thrilled to receive the stroller. I can't wait to take my new little one for walks to the park!")
A thank you for attending the gathering, if applicable
Hope that helps!