DSiWare had a very poorly designed shop front, but there were some very decent games available through the service!
I would definitely recommend Alpha Bounce, Alt-Play Anthology (although this won't appeal to everyone), Art Style Pictobits, Dark Void Zero, Pop Island, Puzzle Quest, Shantae, Starship Defense, and Undead Storm - although it's a shame that last one didn't support download play for multiplayer modes.
Are all of those games available on 3DS eShop? Most probably are... I never really checked to confirm.
The version of Tetris Party Live that is no longer available was fantastic and priced very aggressively - shame it's been replaced by more expensive, but less well put together, versions.