I LOVE this tool for preschool pre-writers: Wood Pieces for Capital Letters from Handwriting Without Tears. It introduces young children to the lines and curves that they will use to write letters when they are ready. It is a tactile, three-dimensional way for them to successfully "build" the letters that they know. And it meets them right where they are developmentally.
I leave our set about in different places (on a mat on the floor, in a basket on the coffee table, on my son's desk) to encourage exploration. Sometimes they become different things too -- this morning he used them to make a rocket ship. Manipulating the lines and curves is fun and he's learning the building blocks of letters as he plays.
I've tried making letter pieces out of paper, oak tag, or cardboard for kids, but there's something about the feel of the wood... I keep coming back to this set. I recommend these for parents all the time. Like unit blocks (another favorite of ours), it's an investment that really seems to pay off.