On top of my head, especially for C/Linux developer:
Is there some tool/script you couldn't work without, but that you feel others don't know so much?
For instance, I just found:
On Windows or Linux:
On Windows (and Linux, Solaris), XMLStarlet
I realize that grep and tail are well-known, but it amazes me how many people think it's magic when you use either one of them. Same thing with Vim. I'm hardly a power user, but I can open it up and have a small document complete before MS Word would even open.
And my other two favourites are too large to be considered small :P
I don't use it that often, but the tool with the highest ratio of awesomeness to being little known has to be dot, which is the tool for drawing directed graphs that is part of the AT&T Graph Visualization suite. A nice example output is their Unix family tree.
It's a language not a tool, but it is small and not yet well known: the programming language Lua. I can't live without it and it is gradually displacing my sh, ksh, awk, and perl scripts... One mark of genius: the escape character for string-search patterns is different from the escape character for quoted strings. No more trying to make sense of legions of backslashes!
The python interactive interpreter and its pumped up variant iPython are the tools that have enhanced my programming productivity like no other.
Being able to experiment with new code on the fly and inspect objects, classes, and modules in any way you want interactively is simply awesome.
Does know one depend on Putty ?, i really dunno what i would do with out it
I need the most tools when coding for web, anything else usually just a text editor.
for web:
Vim and PrintFile. All PrintFile does is print text and PostScript files, any which way I want them (including cut-and-paste).
Most have been listed here already so I won't repeat. Here are a couple new ones.
psValidation (My own Validation Class Library, Mostly RegEx)
Already listed but I must mention TextMate. I use Mercurial or Hg on a daily basis.