Ok so, back in the day when the ds was young, i purchased one. i loved it. the design of what it could do rocked, and mario was awesome.
Although. It was chunky, ugly, and had less games than a microwave oven.
when i looked into it, there were no new ds games planned, and metroid kept getting postponed over and over and over so i just traded the fucker in.
fastforward a year or so, and the DS Lite was released.
when it was announced, i thought it was brilliant. the same package, but smaller, neater, better screens, and much more aesthetically pleasing. i wanted one, but didn't see a need, and i had other things to be playing on.
Recently, my friends have started buying DS Lites.
one of my friends bought a little chip from the internet called a M3 Simply.
he showed me it, and the next day i bought a DS Lite. No kidding. i would have bought one right then and there but the shops were closed. infact, i bought my m3 then and there on the spot.
The M3 acts as a loading machine, user interface and suchlike for loading up DS homebrew, applications and suchlike onto your ds.
using a rather cool Micro SD card as a memory card, you can get loads of games and applications, and even movies, music, and photos and play them on your ds.
and yes the micro sd card is micro -

if you feel inclined to you could get another device, back up your ds games to your pc into a rom style format then place them on the card and load them from there so you only have to carry your ds with the m3 in it, and no other carts. you can even still use nintendo wfc.
micro sd cards go up to 4GB but the 4GB cards tend to have problems at the moment, so its safer (and cheaper) to buy 2GB cards, which are infact, peanuts. you can pick them up for around $15USD and the m3 card costs about the same as a single game cartridge.
there are many different types of cart for interfacing with the ds, the M3, M3 simply, R4, and loads of them ive never tried.
there are rediculous amounts of ds homebrews and apps, from VOIP chats, to FTP clients, to MSN clients and remote pc usage software (control your pc with your ds? mint!) and many others that take full use of the wifi connection in the DS.
if you get a ram cart, you can use ported opera browsers to surf the internet on your ds. pretty nice.
There are megadrive/genesis gamegear/master system emulators, snes emulators and gb emulators.
its just insane what you can get on the internet for your ds.
i won't go into it all, just type in "DS HOMEBREW" on google and look around.
GBA movie player- The ultimate game flash card & movie player for GBA, GBA SP, NDS & GBM!
if you read reviews around the internet, they mostly say "if you have a ds, get one" i say, get a ds and get one. when i ordered mine, there was a global shortage. the factory has backlogs of orders. this tends to happen alot with the m3 and r4.
get yourself one.