One of the greatest add-ons for the DS or DSi is an R4i card that allows you to run homebrew (i.e. non-commercial) software allowing you to play movies, MP3s and read e-books etc.
As a by-product, it also allows you to run back-ups of your games from a single Memory Card. I have already lost game cards in the past (well, the kids did, not me) and this makes thing a lot less worrying as there is no swapping of cards.
The R4i DSi Adapter is discussed and reviewed further at R4i-DSi-Adapter who also have a further 10% discount code promotion that is ongoing.