If you do a lot of writing, a desktop dictionary tool can be a powerful asset. With it, you can highlight a word within any program and then click the dictionary icon in your menu bar, which will open a new Web browser window containing the definition of the word.
To add a desktop dictionary to your Ubuntu installation, use the Googlizer applet, which can be installed using Synaptic Package Manager. This applet is designed to perform instant Web searches using Google, but you can tweak it to perform dictionary searches instead.
Once you've installed Googlizer in Synaptic, drag its icon from the Applications, Internet menu to a spare spot on the panel bar. Then right-click it and select Properties.
In the Command field, change the line to read as follows: googlizer --url http://www.merriam-webster.com/dictionary/. (note that's two dashes before url). How it works is that the search term you higlight is automatically added to the selection buffer, and then simply added to the end of the URL you quote when you click the Googlizer button. In the case of the Merriam-Webster site, this works perfectly - a typical URL to lookup a word on the site would be http://www.merriam-webster.com/dictionary/orange, for example, which will provide a definition of the word "orange".
You might also want to change the icon for Googlizer, like I have, to that of a dictionary. The icon I've used lives at /usr/share/icons/gnome/scalable/apps/accessories-dictionary.svg.