I am a big fan of AddOns, and I loved the WoW AddOn system. Not just using them, but creating them as well. As a programmer it was a piece of cake to learn it.
Rift has a similar system. My only problem that it is also based on Lua, which is a language that I've never grown to like, even after using it for a while.
Still, as I am now playing Rift, I wanted to look into AddOn creation a bit, and come up with one or two ideas.
At this moment it is exactly two:
The first one is an inverse shopping list: You can drag&drop items to a "bin", and when you interact with a shopping window, it opens the - sorted - list, and gives you a button to sell all. This is basically an extension of the already existing [Sell greys] button. A lot of additional functionality could be included in this one, such as an actual shopping list with items that you need to buy from time to time, etc. But sadly, it can not be done with the current Rift API, because it doesn't allow you to interact with shops. Probably a thing to come.
The second one, on the other hand, seems to be perfectly viable: A raiding/grouping minimap, that shows the players/pets/enemies from a top down view. There is just one problem with that: I am lazy, and as I do not group often, I don't see a reason to create it. (nor have the actual knowledge to make it really useful)
But that is not the end of it. I have a few other half formed ideas in my head, some of them might actually come true. I'll give you an update then.