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/' *

for dry-run, use -n option!

A graphical bulk renaming tool is:

sudo apt-get install pyrenamer