I see that Mac of Mulier Fortis (also a science teacher) has recently posted on a song about DNA, which provoked some interest from Fr. Tim, who wishes he had had this sort of thing in his science lessons when at school.
Not to be outdone, here is one of my favourites for introducing the topic of elements and compounds in chemistry. The animation includes a surprising amount of information about chemical elements and the periodic table, and is also useful for revision - get the class to identinfy and explain as many points as they can find shown in the video
The video comes from a DVD called 'Here Comes Science' by a band called 'They Might be Giants'. There are quite a few very good videos/songs on the DVD for giving overvies of science topics (sadly, the opening track, called 'Science is Real' appears to be a bit of propaganda for the Dawkinsian materialists, but fortunately it is easy to skip / ignore / never show to a class).