Some browsers have their own options for dealing with offensive content whilst surfing. Some ISPs also offer filtering of content and block sites that may be offensive, including AOL and UK Online along with some of the Christian ISPs (see the list at OSCAR - Supporter zone - Communications). Occasionally this filtering can be selected or deselected by password from the user (adult) to protect children from such content. There are also 'safe' search engine options that filter out bad content in search returns. These can sometimes be switched on or off when you search.
There are also various stand alone filtering software programmes around (for both PC and Mac), some of which work reasonably well. Others, though, can have trouble distinguishing the good sites from the bad and prove to be more frustrating that functional. Here are some of the choices:
There are also various online services (some Christian) that work as a proxy to filter content. This list below of mostly US based services is provided by Brigada:
The best advice is that if you are concerned about access to offensive content (in particular with your children) then make sure that something is between the user and the content, whether it's settings, ISP filters, special software or even someone supervising.