Fisheye lens: Samyang 8mm or Sigma 8-16
There's also the Sigma 10mm F2.8 EX DC Fisheye HSM which I have and love. I bought it when it first came out and if I'd known about the cheaper manual focus fisheyes I'd probably have bought one of those instead if money was an issue. And I also have the Sigma 8-16. So it depends on if I want to use a true fisheye or just an ultrawide.
The Sigma 8-16 gets more use especially since I have a ziekos o.18x fisheye adapter for my Sony 18-250 lens. The Zeikos is a full circular fisheye and surprisingly very fun to use for under $50 including step-up ring.
link to ebay ziekos seller http://compare.ebay.com/like/200721647499?var=lv