Please find below a list of ABAP demo programs provided by SAP which can be used for reference purposes:

ABAPDOCU transaction : ABAP Documentation and Examples

LIBS transaction : Collection of Ideas for Table and List Design

BIBS transaction : Style guide

DWDM transaction : Demo Examples in ABAP Objects (control framework). The demo programs are also stored in SDW4 package

Flight Data Application : Demo Example for Integration Technologies: an application which demonstrates many technologies for example BAPI, BOR, ALV, Smart Forms, etc…

SLIS package : ALV demo programs (mainly BCALV_TEST*)

ADBC_DEMO program : Demo of ABAP database Connectivity (to connect to another database)

SE83 : SAP Reuse Library

We will try to keep this list updated.