The GoPro's lens has a strong fish eye effect. This can easily be converted to a more 'natural' look with the free panorama stitching software Hugin

1. Start Hugin and go to the assistant tab
2. Load your image (load images button or drag & drop)
3. Select 'Circular fish eye lens': focal length, 16mm multiplier 1. EDIT: the best value for the GoPro HD2 appears to be 15.6
4. Go to the stitcher tab
5. Select cylindrical projection
6. Press 'Calculate field of view' and 'Calculate optimal size'
7. Press 'Stitch now'

The horizon can be leveled by adjusting the projection in the 'preview panorama' window (icon in tool bar or <ctrl> + p). This action can best be performed between steps 5 and 6 in the previous section.

1. click the point that you want in the center with the left mouse button
2. click on another point that must be horizontal with the first point with the right mouse button

The final crop can be made with your preferred image editor. I like Gimp, the poor man's Photoshop.

For more examples: see this page