I agree Jenny Hart's Sublime Stitching is amazing and a great place to start!
There's a new site I've read about on a couple of crafty blogs that has really cute embroidery patterns. They are called "Pimp Stitch" but don't let the name fool you, they are definitely g-rated. You can find them here:
http://pimpstitch.typepad.com/A lot of stitchers share their vintage embroidery patterns on Flickr. Do a search for embroidery patterns and you'll find many patterns you can download under the creative commons license. Vintage coloring books are also a great place to find images suitable for embroidery.
As for creating you own just trace an image with a transfer pencil (under $2 at any craft store) and iron it on to the fabric and start stitching. You are only limited by your creativity! Just remember when you transfer the image to the fabric it will be a mirror image of your design. If you want to embroider text you will need to reverse it before ironing on to your fabric.
Happy Stitching!