Here are some Haskell logo i made. I hope webmasters will use it to spread Haskell. I hereby release it into public domain.
The official logo, of course, can be found at http://haskell.org/
For the horizontal version, it is also possible to display it without using image, by using HTML, unicode, fonts, stylesheet. Here's the code and how it displays in your browser:
<div style="display:table; border: solid thin black; text-align:center; padding:1ex"> <span style="font-size:large; font-family:sans-serif; word-spacing:2ex">λ → ∀ ≫ :: ⇒</span><br> <span style="font-size:xx-large; font-family:serif;">Haskell</span>
you should also add the unicode declaration right after the <head> tag in your HTML:
<META HTTP-EQUIV="Content-Type" CONTENT="text/html;charset=utf-8">