Generally the preschool years don't require too much formal instruction. However early childhood education is very important because it sets the foundation for all the future learning that a child will do. It also sets the foundation psychologically and emotionally and will effect the child's outlook on life, so it's crucial for parents to invest a lot of effort at this time in their child's development. Most of the learning at this stage takes place just doing the regular everyday things. And many basic skills are learned just fine through interaction with parents, siblings and friends, through a lot of play, through reading and discussing, looking closely and analyzing what one sees. Children develop and mature at different rates so instruction of any sort has to be appropriate for the particular child. But with a lot of love, patience, flexibility and enthusiasm, a tremedous amount could be accomplished during this stage. Here are a few links with some useful stuff for preschoolers.
www.first-school.ws -lots of worksheets, games, crafts and more for young children
www.letteroftheweek.com - a preschool curriculum with lots of interesting things, I really like the journal section. There are also good ideas for introducing science and history among other things.
www.carlscorner.us.com - more stuff for teaching basic skills