SamuelCB, you may find these threads interesting:
http://arduino.cc/forum/index.php/topic,44427.0.html
http://arduino.cc/forum/index.php/topic,59003.0.html
http://arduino.cc/forum/index.php/topic,88459.0.html
The long and short of it is, most 16x2 and 20x4 character-based LCD displays will let you define up to 8 custom characters.