1. lsof
  2. pgrep/pkill
  3. apt-file
  4. traffic monitoring
  5. whowatch

lsof

package: lsof

lsof can do all kinds of cool stuff, including tell you which processes are using a partition (so you can kill them and unmount the partition), and tell you which processes are bound to a particular port.

processes using /var:
# lsof /var

processes bound to port 80:
# lsof -i 80

pgrep/pkill

package

apt-file

package: apt-file

Search the entire debian repository. This can be useful to find what package a certain file comes from when you don't have that package installed.

> apt-file search <filename>

traffic monitoring

iptraf
ethstatus

whowatch

browse through users and the tree of their processes and tell what those processes are doing. cool.