Some useful table names for Basis:

TSTCT
List of Transaction Codes

TBTCT, TBTCO, TBTCP
Background Jobs data is stored in these tables

USR02
Logon data

USR04
User master authorization (one row per user)

UST04
User profiles (multiple rows per user)

USR10
Authorization profiles (i.e. SAP_ALL)

USR11
Text for authorization profiles

USR12
Authorization values

USR40
Table with list of password restrictions and prohibitions

AGR_USERS
List of roles assigned to Users

AGR_T-Codes
List of all T-Codes assigned to Users

E070, E070A, E070AS, E071
Transport data is stored within these tables

TST01
Table for TemSe

PAT01
Table for Support packages and updates

PAT03
Status of Support packages and updates

T000
List of all the Clients

SNAP
ABAP dumps are stored here

TPFYPROPTY
Table with list of all dynamically switchable parameter