Some useful Data Dictionary Views

Table spaces
dba_tablespaces
user_tablespaces 

Tablespace Quotas
dba_ts_quotas
user_ts_quotas

Data Files
dba_data_files
v$backup_datafile
v$datafile
v$datafile_copy
v$datafile_header

Free Space
dba_free_space

Segments
dba_segments
v$segment_statistics

Extents
dba_extents

Blocks
v$database_block_corruption

Groups
dba_tablespace_groups

SYSAUX Tablespace
v_$sysaux_occupants

Temp Tablespace
dba_temp_files

Undo Tablespace
dba_rollback_segs
dba_undo_extents
v$rollstat
v$undostat

Transportable Tablespaces
transport_set_violations