Add prefix to filename:

rename 's/^/MyPrefix_/' *

Remove part of filename:

rename 's/SEARCH/REPLACE/' *

Add suffix to filename:

rename 's/$/_MySuffix/' *

Rename extension:

rename 's/\.pdf$/.doc/' *

Swapping parts of filename:

rename 's/(.*) - Linkin Park (.*)$/Linkin Park - $1 $2/' *.mkv

for dry-run, use -n option!

Mass moving files from numbered directories into another one (bash for loop):

for i in `seq 2 30`;do mv bla$i/* target_directory ; done

A graphical bulk renaming tool is:

sudo apt-get install pyrenamer