I'll second Mario Party 8 - while our 4 year old lacks strategy, she is able to perform the controls for all of the minigames. The down side is that a 10 turn game tends to take us about an hour, so she only gets to play 1-2 games/week. We rented monkey ball, but she lacked the finesse to play it.
She also enjoys the find mii and cow racing parts of Wii Play, and Super Mario Bros (VC).
We've found the photo channel to be very kid friendly - we load pictures the 4 yo takes (digital cam) and both like to doodle/paint on them (4 and 2 yrs old). I've read that the Wario game is kid-friendly, but haven't tried it.
Probably ~80% of our Wii on-time is us leaving the slideshow running (photo channel - up to 1,000 pictures on an SD card) while we do other things - nice way to convert a TV to a digital picture frame.
There may be more out there - especially on VC, but we don't have the time for it (we keep their Tube-time (TV + PC + Wii) to ~2 hrs/day).
As far as PC games go, we like the Blue's Clues series.