I love the magnet like slopes - it's one that just makes you go "wow". Reminds me somewhat of the James Dyson "Wrong Garden" illusion as discussed previously on HN (http://news.ycombinator.com/item?id=779555, comments of thread discuss how to see a short video of it).
The "counter-intuitive illusory contours" one is interesting. It is almost as though our brain is treating the image as a liquid, and imagining distortion waves ahead of the "collision point".
The sine wave one is interesting in that you no longer notice the effect if you are reading the text below it (i.e not focusing on the wave beyond your periphery). This is somewhat similar to the "spinning dancer" illusion (http://en.wikipedia.org/wiki/The_Spinning_Dancer) in which you can trick your mind to see the illusion in different ways, and sometimes have difficulty seeing it again as you did originally.
In my related searches, found this site: http://yloveillusions.com/en/category/uncategorized/page/5/ which has some other interesting illusions. Particularly like the "A bottle disappears" one.