Character Map
With Character Map, you can select a large range of non-standard and international characters to put in your documents.
Menu: Accessories > Character Map
Key Features
- Choose from many different international categories
- Includes support for the phonetic alphabet
Screenshot
 
 
Installation
- Part of the default Ubuntu installation
Hints and Tips
- View >By Unicode Block can make it a lot easier to find mathematical symbols. 
- Search find dialog is also quite useful if you want to copy a particular symbol as well and searches over the names.
Other applications you may wish to look at
- Character Palette applet 
- ComposeKey direct entry by typing multiple keystrokes