Here is a good site with many useful conversion tables

http://www.simetric.co.uk/si_lm.htm

Here's another one. Not as wide ranging as that one but has most stuff you need regularly.

http://www.citroen.co.uk/conversion/

you can download it as well. I've got it as a shortcut on my desktop at work.

Try this programme. I'm an engineer and use it everyday for metric/impreial. It has every measurement that you could ever want and being a small file, stays open all day on my desktop.

The best one? (Well IMO)

www.onlineconversion.com

Converts just about anything to aything else. Some good old units in there as well

Same here I use onlineconversion.com