Timo Glock helmets:

Helmet designs are more than just a pretty design on carbon fiber, they are of sentimental value to a drive, a hallmark of their career which symbolizes their personality.
The two cards on the back of Alonso's helmet refer to his two titles - and also on his passion for playing cards: 

 

There are more tricky and less obvious ones though. For example, Michael Schumacher had some Chinese signs on his helmet. Those were the names of his family members. Here it's Mick - his son - (above the AMD sign) and Michael, on the other side it's Gina Maria and Corinna (his daughter and his wife). 

 

This is Gina's name: 

 

Michael has 7 stars on the top of his helmet, but those are NOT to represent his titles. They were there since his Benetton times (although the number of the stars varied at the beginning, there was five and I think even four, but he was still at Benetton when it became 7). IIRC he once commented he just likes the starry night sky, so that's why it's on his helmet.