mario kart is the biggest one with online but is a bit family.
excite truck gets good reviews but no online.
gyrostarr is a wiiware game that you can download so it costs less thats supposed to be ok. not sure if its out yet but no online...but you can compare your times against other wii users online.
NASCAR kart racing comes out feb 10 but idont know if its online. also the graphics arent like real life nascar. its more cartoon.
thats all I know. maybe someone else knows.