I have a collection of aliases, shell scripts, and small perl scripts that are in my default path. They're simple, but they save me some work so I use them a lot. For example I have a command called "sl" that expands to

ls -halt ... | less -E
[download]

I do most of my work on a couple of unixes, all of which either share the same home dir or have copies of my scripts. When I occassionally have to work on a different server, I drive myself a little crazy because I keep typing these non-existent commands. So I guess that makes them tools that I can't live without :)

update: I just put the sl script into the code catacombs here: expanded "ls".