Here are some of the sites I use most often for ideas and supplies:
I recently discovered that Lakeshore Learning has some great free online resources at their Teacher's Corner. This site includes craft ideas, simple lesson plans (that you can print out), printable worksheets, customizable writing practice sheets and word searches.
Starfall's Learn to Read with Phonics
This is a great, free online resource for kids learning to read. For preschoolers, start with the ABCs link. It teaches phonics and you can also get free letter activity sheets to print out. There are no licensed characters and the site really is free, no registration required.
The lessons I do are typically science-based. Although I keep them simple for 3-5 year-olds, I like to do some research online to augment my knowledge on a subject. I often start with a Google search, but more often than not, I end up on Wikipedia.
"WPClipart is an ever-growing collection of artwork for schoolkids and others that is free of copyright concerns as well as safe from inappropriate images." I frequently use clip art to get photos to print out for preschool, to create PDF docs with photos to show on my Nook, or to create my own worksheets if I don't find something that goes with my theme in one of my preschool workbooks.
S.B. is a stay-at-home mom with young children and has some great original craft ideas suitable for preschool age kids.
A great place for buying craft supplies in bulk. You often have to buy more than one of an item, but the prices are so great, it's worth it. I buy a lot of preschool supplies on Amazon, but for basic craft supplies, CreateForLess comes out to be a better deal, even factoring in shipping.
Rachel Rambach on YouTube
Rachel is a teacher and posts weekly songs to sing with children. Some are original and others are ones she has adapted from other song writers. In addition to singing the songs, she explains how she uses them in the classroom.
Preschool Express by Jean Warren
She has some great original content and the site is easy to navigate. I often use Jean's website to find songs or rhyme's for my lessons based on my theme and love that I can search content based on theme.
The Perpetual Preschool
I'm not crazy about the layout or ads on this site, but there are some good community-submitted ideas for songs on various themes.