The starter of this whole universe
SAP AG: www.sap.com
I would mention more than one link here. The place you find standard help: help.sap.com. Some important standards for programmers: Function keys

4 Abap Programmers
A nice undependent site with lots of content. It is written in german.
www.4ap.de

SAP Img
Some strange content here, maybe unapproved by SAP.
www.sap-img.com

Abap Fans
You can ask your questions in German and they will be quickly answered.
www.abap-fans.de