1. Amigurumi Pi ornaments. I admit to loving amigurumi, I wish I could do it and there are so many cute designs out there. Any little amigurmi can be an ornament. These were made by alicia954.
2. When in doubt go with Dr. Who. Polymer clay is a great media and you can make your own Tardis, or my favorite, the Dalek. I have seen many weeping angel tutorials on how to turn Barbie into something terrifying (well at least more terrifying than she already is) to put on top of your tree. This Dalek was made by headofheather and she has many cute geek ornaments for sale at her Etsy store.
3. For the science nerd we have these cute periodic table ornaments. A flat blank ornament, paper mache or wood. Some white paint, stick on letters, a little santa hat, and you are ready to create your very own ornaments. These can be found over on Artfire if you wish to purchase instead of making your own.
4. Pokeman is very popular and I found this round ornament made from sequins for sale on Etsy. If you are a very patient person you can take any round idea like Angry Birds, the Death Star, or even logos from your favorite game like Zelda or WOW (Alliance or Horde) and create your own.
5. And last but not least using those old circuit boards. Cut them in cute themed shapes, punch a hole and you are done. Recycling at its best. You can the same with the old CD's to decorate or just cut in great shapes.
So have a very happy holiday and let your tree truly show who you are.