A
website displaying an image considered by many to be extremely
gross - namely, of a guy spreading open his
anus to impressive dimensions. Especially the effect on people with even rudimentary
homophobia is obvious.
It was at one time a very fashionable game on slashdot and other online discussion fora (especially on games) that allow the usage of HTML to write postings that pretend to link to some interesting site, but in fact mallink to http://goatse.cx. This in turn has resulted in the expression goatse to become a widely-known meme. In fact, "goatse trolls" became so pervasive that Slashdot had to (AFAIK mainly for this reason) add a feature that appends the domain of a link URL to the link's text. The trolls reacted by finding ingenious ways to have people redirected to goatse.cx through various respectable-seeming domains.
Nowadays, "goatse links" are not anymore commonplace but are strongly remembered and still crop up from time to time.