SpudFiles

http://www.onlineconversion.com/pressure.htm

convert from everything to everything.

one good tool for a spudgunner.

Or, download http://joshmadison.com/article/convert-for-windows .
It'll convert pressure, volume, length, speed, temperature, work, energy, and many other units.

i have that one (i think) but online is better for some people

I just use goolgle:

If you enter 100 psi -> bar google will say
"100 pounds per square inch = 6.89475729 bar"

This works with almost any unit you usually come across.
The only downside it that its picky with stuff like ft/lb's or something like 2", you need to write down foot pound and 2 inch instead

i also use google..
but i think this one looks more professional 8)

jimmy: the one you suggested are really good.

How bout installing units command for terminal in linux, you can convert milimeters per picosecond to furlongs per fortnight
Nice find for the non linux users.