Transaction You Will Probably Use Most

SE80 – Object Navigator
SE80 is the most important transaction for the ABAP programmer. It's the entry point for the workbench. From SE80 you can for example create new packages, classes, programs, dictionary objects (database tables) etc.

SE11 – Dictionary
Using SE11, you can create and maintain database objects such as Database tables, data types, and lock objects.

SE30 – Performance
SE30 is used for testing performance. Want to compare two bits of code that do the same thing and see which one runs quicker? SE30 is the place to do it.

SECATT – Extended Computer Aided Test Tool
Extended Computer Aided Test Tool (or ECATT) is a built-in tool to test SAP system. By using eCATT you can:
• Test transactions, reports, and scenarios
• Call BAPIs and function modules
• Test remote systems
• Check authorizations (user profiles)
• Test updates (database, applications, GUI)
• Test the effect of changes to customizing settings
• Check system messages

SU01 – User Maintenance
This is the place to maintain user data such as personal data, roles, logon data (passwords) etc. It's kind of like SAP's Java WAS's User Management Engine (UME).

SE81 –Application Hierarchy
SE80's less famous sister, SE81 allows you to browse SAP applications. Clicking on a node (application) on SE81 opens that application in SE80.

SE91 –Message Classes
SE91 allows creating and maintaining message classes.


SM12 –Lock Entries
Using this transaction you can see which lock objects are used by which user for a table of your desired. See the postregarding lock entries for more information...

SM30 – Maintain Table Views

Using this transaction you can maintain table views, manually adding, editing and removing information from existing tables for which you created a table view.

other

Useful Dev. and Workbench Transactions
SCAM CATT Management
SCAT Computer Aided Test Tool
SE09 Workbench Organizer
SE10 Customizing Organizer
SE11 ABAP/4 Dictionary Maintenance
SE12 ABAP/4 Dictionary Display
SE13 Maintain Technical Settings (Tables)
SE14 Utilities for Dictionary Tables
SE15 ABAP/4 Repository Information System
SE16 Data Browser
SE17 General Table Display
SE30 ABAP/4 Runtime Analysis
SE32 ABAP/4 Text Element Maintenance
SE35 ABAP/4 Dialog Modules
SE36 ABAP/4: Logical Databases
SE37 ABAP/4 Function Modules
SE38 ABAP/4 Program Development
SE39 Splitscreen Editor: Program Compare
SE41 Menu Painter
SE43 Maintain Area Menu
SE51 Screen Painter
SE54 Generate View Maintenance Module
SE61 R/3 Documentation
SE62 Industry utilities
SE63 Translation
SE64 Terminology
SE65 R/3 document. short text statistics
SE66 R/3 Documentation Statistics (Test!)
SE68 Translation Administration
SE71 SAPscript layout set
SE72 SAPscript styles
SE73 SAPscript font maintenance (revised)
SE74 SAPscript format conversion
SE75 SAPscript Settings
SE76 SAPscript Translation Layout Sets
SE77 SAPscript Translation Styles
SE80 ABAP/4 Development Workbench
SE81 SAP Application Hierarchy
SE82 Customer Application Hierarchy
SE84 ABAP/4 Repository Information System
SE85 ABAP/4 Dictionary Information System
SE86 ABAP/4 Repository Information System
SE87 Data Modeler Information System
SE88 Development Coordination Info System
SE91 Maintain Messages
SE92 Maintain system log messages
SE93 Maintain Transaction Codes
SEU Object Browser
SHD0 Transaction variant maintenance
SQ00 ABAP/4 Query: Start Queries
SQ01 ABAP/4 Query: Maintain Queries
SQ02 ABAP/4 Query: Maintain Funct. Areas
SQ03 ABAP/4 Query: Maintain User Groups
SQ07 ABAP/4 Query: Language Comparison