You have the following new options: Pentax DA 14mm f2.8
Samyang 16mm f2.0, Samyang 14mm f2.8, Samyang 10mm, Samyang 8mm FE (many Samyang lenses are also sold under many other names: Walimex, ProOptic, Falcon, Rokinon, Bower, Vivitar)
Zenitar 16mm f2.8 FE, Peleng 8mm FE (these two are available in m42 and K-mount, I think. Can still be found as new from Eastern Europe)
Zooms:
Pentax 12-24mm, Pentax 10-17mm FE
Sigma 10-20mm, Sigma 8-16mm, Tamron 10-24mm
What I would look at is, how does the DA 15mm not fit your needs? And then find a lens that does that well. Do you need faster aperture? Do you need AF? Maybe a fisheye is your best bet - if you shoot raw you can use software to de-fish it and still get an "ultra wide" look.
My Samyang 14mm is quite wide enough for me. In fact, I often think of switching it for 16mm
Originally posted by zzeitg Just came accross Samyang 10mm F2.8 - any good? How does it differ to Sigma 8-16?
In many ways. Samyang is prime, Sigma is zoom. Sigma is very highly praised, but it has its weaknesses and it is a fairly big lens. Check the Lens review section. The Samyang, according to reviews, appears to be quite great, but manual focus