Cassandra Clare managed to sell City of Bones with only 10 chapters written, along with an outline for the rest of the series. What's stopping you? Get writing! (Source.)
Think Jace's ring would look great on your favorite guy (or on a chain around your own neck)? You can buy one, and other Mortal Instruments merch online. (Source.)
Many of the book's serious fans hoped for Beastly star Alex Pettyfer to play Jace. There's even a Facebook fan page for it. And an online petition. Alas, that ship has sailed, and it looks like Jamie Campbell Bower is locked in for the role.
WARNING: Painting these on your skin may or may not (probably not) give you heightened powers. Check out the Shadowhunter runes here.
Ever wanted to sneak a peek at Gandalf's diary? Cassandra Clare (writing as Cassandra Claire, with an 'I') wrote the very popular fan-fiction The Very Secret Diaries, a comical look inside the private lives of the Fellowship of the Ring.