We love region-free consoles! Not only does it allow us to play games that never get released in the US, but it also gives us freedom to choose the cheapest version of worldwide-release games. Renchi.com has marked down a load of DS games, and there's plenty of stuff for US gamers to enjoy.
First, the Japanese versions of US games. You can get Yoshi Touch & Go (Catch! Touch! Yoshi!), Mega Man ZX (Rockman ZX), Star Fox Command, the just-released Honeycomb Beat, and a few others for $10. If you want a uniquely Japanese experience, there's a bunch of stuff including Mawasunda!!, a lot of miscellaneous non-games and puzzle games (including an amusing-looking coloring game), and a nice selection of D3's Simple Series stuff. Stock is rapidly diminishing, so go browse right now.