How can I stream my iTunes library to my iPhone or iPad?

If you have iOS 4.3.4 go to settings>iPod and use your Apple ID and sign in. Anytime you are on your home network you can see it in the iPod or Video app. There are other apps that will let you stream via the internet. You can find them in the app store.

Home Sharing:

http://support.apple.com/kb/ht3819