Merlin, a television series in the UK that also plays on SyFy in the US, will be having a few Harry Potter actors guest star in Series 3!  As I posted a long time ago, John Hurt (Olivander) has been doing the voice of the Dragon since the first episode.  This season we get to see Miriam Margolyes (Professor Sprout) and Warwick Davis (Flitwick, Griphook). According to Colin Morgan who plays Merlin:

Any special guest stars in the third series that you won’t get into trouble for talking about?
I’ll probably get whipped for it later, but I’ll be fine. [Laughs] Fans of the Harry Potter films will see a few familiar faces. One of the great people who we just had recently was Miriam Margolyes, who features as Professor Sprout in the Harry Potter films, and she is just an absolute blast. That’s an episode in the third series, the sixth, to look forward to. And one I’m really excited about, and he hasn’t been in yet, is Warwick Davis. I’m a huge fan of Willow from growing up, and he’s featured in Harry Potter.

This is one of my favorite shows, so I’m quite excited!