I'm a maid of honor for my sister's wedding in April. Other than buying my dress and shoes, I am also paying for an artist to come to th ehotel and do our makeup and updos on the day of the wedding. I am also in charge of her bachelorette party, and bridal/lingerie shower. You don't have to necessarily pay for the parties, but you are in charge of organizing it and contacting the girls,getting them to chip in,etc. Expenses should be shared equally among the girls. Instruct them to save all receipts for decorations, gag gifts, shower invites, etc. As maid of honor, you'll be in charge of tallying totals and hashing out who owes what.
You are also expected to make a toast at the wedding reception and to wait on the bride hand and foot the day of the wedding. Its a tough job believe it or not!!
Here is a site that explains your duties as maid of honor:
http://www.weddinggazette.com/content/002309.shtml