Hold the left key and type 168 on the right hand keypad.
There are a number of these available.
| Character | <alt>+ |  | Character | <alt>+ |  | Character | <alt>+ |  | Character | <alt>+ |  | Character | <alt>+ | 
| á | 160 |  | à | 133 |  | â | 131 |  | ä | 132 |  | é | 130 | 
| è | 138 |  | ê | 136 |  | ë | 137 |  | í | 161 |  | ì | 141 | 
| î | 140 |  | ï | 139 |  | ó | 162 |  | ò | 149 |  | ö | 148 | 
| ú | 163 |  | ù | 151 |  | û | 150 |  | ü | 129 |  | ç | 135 | 
| Ç | 128 |  | É | 144 |  | « | 174 |  | » | 175 |  | ñ | 164 | 
| Ñ | 165 |  | ¿ | 168 |  | ¡ | 173 |  |  |  |  |  |  | 
For Mac's the OPTION + the mark + the letter.  Use shift to make it captal.