Improved my typing speed. For some reason, there's a correlation between me playing osu! and then me getting high WPMs on typing tests taking immediately after a good osu session. Maybe it's a benefit from the increased concentration I have to dedicate towards osu, which carries over to the typing test?
In all seriousness though, osu has improved my concentration and attention span by quite a bit. Multiplayer is it's own personal hell for me (I always seem to choke up at the most inconvenient moments)