Here's a few of my images where I've broken the thirds rule, and why I think that they still work, compositionally.
This is the Salar de Uyuni in Bolivia's Altiplano. The horizon is much higher than the top third line, and there's nothing distinctive along the third lines or intersections. Ignoring the third rule can work well with repeated patterns and if there's strong perspective that leads you into the shot:
Sometimes including more sky can add atmosphere to an image, although this can be quite subjective. This is a shot of icebergs on Jökulsárlón Beach in South-East Iceland. Although there's a huge amount of sky and the horizon is right across the middle, the detail is still around the lower horizontal line, so it's only half breaking the rules:
Reflections in water are a great example of where a horizon placed bang in the middle adds drama to the composition. Refections make striking images because of the symmetry around the middle. This shot was taken in Kulusuk in South-East Greenland:
Here's another symmetrical reflection shot, taken along the canals near Bruges in Belgium (on a point-and-shoot camera) - there is nothing distinctive along the thirds lines or intersections, but it still makes a dramatic shot, with the eye drawn towards the centre point (it does have leading lines and diagonals, which are also suggested in composition for a successful shot):
Another reason to break the horizon rule is if you are shooting for a book or magazine cover, where some room is needed for a title across the top. A horizon level between the top third and half way line usually provides enough room. While it might not make a great photo on its own, with the text overlaid it could be the perfect composition for a magazine cover. (Tip: it's worth shooting a variety of shots of the same scene - with normal composition (using the thirds rule), leaving some room for text, trying different orientations, etc..)
See also Breaking the Rules Part 1 - Go Out in the Midday Sun
Part 3 - Turn it Upside-Down!
Part 4 - Shooting Landscapes in Portrait Orientation
Part 5 - Playing with the Zoom
Part 6 - Shooting Out of Focus