As well, if you post about the contest on your blog you will receive another chance to win.
What is the 100th cartridge? It is the brand new and fabulous "Winter Wonderland" cartridge. You can find out more details about the cartridge here. This is definitely one I hope to add to my collection very soon.
The thread for week one of the giveaways can be found here. Good luck to all those who enter :)