The site http://www.sciencelab.com/data/conversion.shtml offers a free and comprehensive conversion calculator online.

A conversion program can also be downloaded at http://www.joshmadison.com/software/

Do you know any other sites with similiar applications that make our life easier?

Google will do simple stuff...

eg 456g in oz ---> will give the oz equivelent of 456 g

onlineconversion.com

Handy Molar Mass and Percent by Mass Calculator: http://stud1.tuwien.ac.at/~e9125168/javas/jmolar.html

Javascript provided too, so you can add it to your website.