It's a kind of magical mouse driver that supports nearly every USB mouse made, including _all_ the quirky little buttons and extra functions. As the author said, it's ultra-light-weight, native Mac UI (none of that weird chrome slop that companies like Razer insist upon using).
I use mine with a funky Razer "MMO" mouse that has a funny numeric keypad on the side — all ten of the keys (plus the normal assortment of buttons) can be mapped any way I like.
It came out all the way back in 2005, and has worked reliably ever since. Flat fee for the license, no subscriptions, free demo. Basically everything we all wish apps were.