You need to check this game out. I have heard from multiple people that it can really help increase your mouse accuracy...
I have played it quite a bit over the last few weeks and it really works, and it is a lot of fun! I particularly like playing the 4 minutes song by Madonna and Justin Timberlake.
Here is how to set it up.
Sign up for their site at http://osu.ppy.sh/
Download the game from said site
Search for a song and download it in the Beatmaps section of the website.
Simply open what you downloaded and Osu! will do the rest!.