I just answered this question awhile ago so Imma just copy and paste..
The DSi is a pretty neat thing mainly cause it goes beyond just a portable gaming sytem unlike the regular DS
Camera
Has the ability to upload those pictures to computers with an SD card, which is personally the best part
Internet
Has the ability to surf the web, and download lil' games and apps.
Who doesn't want a portable pocket sized sketchpad

?
http://i141.photobucket.com/albums/r62/Z…
Check that out. Drew it on mah DSi. DS can't do that
Music
Has the ability to play as much music as you can fit on an SD card
Also has this lil' feature where you can dink around with the music, by making it go slower, faster, lower and higher pitch
It's a pretty cool thing to carry around. I probably use its apps just as much as I play games which brings me to my conclusion - if you just wanna play games, and/or don't need another camera/music player, a DS Lite is for you. However, if you want a camera, music player and portable internet surfer for just thirty bucks, get the DSi