Do you knit? Do you have a large stash you aren't sure what to do with? Do you just want to help? Here's a fantastic project:
Knit-a-Square takes donations of knit and crocheted yarn squares from around the world. They are then assembled to make blankets that are given to orphanages in South Africa. All you have to do to help keep a child warm is to make one or more squares and send them.
Crochet and knitting patterns are available on the site along with the address where you can send your squares to.
If there's a yarn shop in your neighbourhood, consider asking them to host a knitting circle. It's a great, simple project that just about anyone can do and a few people can make a ton of squares in very little time.