Ballistic NG is a free, open-source WipEout 'clone' that's currently getting a lot of praise.
It's a take on the early WipEout trilogy, and it nails it SO HARD, that it's basically WipEout 4. Track design is amazing, the controls are elements of WipEout/2097-XL/Wip3out, all combined together, you can import ships from the first three WipEout games into it - it's basically the game anyone keen on the series has been waiting for. Even the visuals take after the original games, complete with PSX texture warping and jittery polygons.
I've been hooked on it for a couple of weeks now, and all I can do is sing it's praises from the rooftops to anyone who will listen.
It's free, and I can't really recommend it enough