It can be done with GNOME.
Peruse the Code Charts page at Unicode.org to find the characters you want to type.
Suppose you want to type the Greek letter mu (micro) with Unicode ID "B5" (hexadecimal).
You would simply need to press Ctrl+Shift+B5: µ
Let's see some other interesting characters.
B9: ¹
B2: ²
B3: ³
B6: ¶
BC: ¼
BD: ½
BE: ¾
A3: £
A7: §
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If you're a C programmer with some spare time, take a look at the mod_virgule project page and help us with one of the tasks on the ToDo list!