Awesome Note ($4) is a fantastic way to organize all the little things you need to purchase for the many events you'll be planning.
Like Evernote, Awesome Note is a planner for everything, but its shopping lists are our favorite. All sorts of information can be added to items on the shopping list. Tags are a good way to sort items by the event they'll be used for (e.g. rehearsal dinner, reception, etc.), who will be using them (e.g. mother of bride, groomsmen), and/or where they can be purchased so you don't have to make multiple trips. These tags, of course, are easily searchable.
Attach a photo, location, or drawing of the item, which is best if you're delegating the task to someone else (make sure they know what you're asking for!). The items can be shared via email or printed out.