BADIs

• Step by Step guide on BADI with Filter implementation (More details) NEW
• Step by Step guide on BADI Multiple use implementation (More details)
• Defining and implementing a BADI (step-by-step with screenshots) (More details)  
• Defining and implementing BADI (With better screenshots) (More details)
• Understanding Business Add-ins (BADI) (More details)  
• Filtering the work items in the SAP inbox using BADI (More details)
• Implementing the BADI for the transactions VL02 & MM02 (More details) 
• Exercise on BADI (More details)  

User Exits

• Implementing Screen Exits for the transaction MIGO (More details) NEW
• Implementing Screen Exit for the transaction CO01 (More details) NEW
• Changing the description of the standard SAP fields (More details)
• Implementing Screen Exit in "CJ01" (More details)
• Adding a custom screen in XD01(More details)
• Implementing Screen Exit for MM01/MM02/MM03 (More details)
• How to implement a screen exit to a SAP Standard transaction (More details)
• Finding user exits using a TCode (More details)
• Working with User Exits (More details) 
• Implementing Field Exit for the transaction MK01 (More details) 
• Adding custom fields using EEWB Transaction (More details)

Also check out the tutorials on:

• SAP Workflow
• LSMW
• XI