Translate Convert is a great tool for Mac OS X. It combines both a unit converter and a machine translator in a single window. There is also a VAT (value added tax) calculator. The main window consists of a toolbar at the top with three buttons and a pane below that changes every time you click one of the buttons. The buttons let you access three features that I've mentioned above and they are "Translate", "Convert" and "VAT". The first feature, the translator, shows you two text fields, with two drop-down menus that you can use to select the source and target languages. You can also use auto-detection of the source language if you want. You can print, save and copy the translated text.
The unit converter, which you should update from the preferences before using it, has a category drop-down menu, which includes angles, area, currency, density, energy, etc. You type in a value and select a unit, and the app shows you the conversion to other similar units.
Lastly, you have the VAT calculator. Here, you specify a VAT percentage and type in a value and the app gives you a final price.
In short, this is a great app that has three nice features in one. You can access it from your desktop but some of the features require an active Internet connection, like the translator, which connects to Microsoft's translation service.