OpenDNS is a free online service that offers an extra layer of safety on the Internet. Technically, the service is DNS resolution, which I'll explain below. The main defensive computing advantage it provides is protection from bad web sites, most importantly from phishing scams. ID theft is, to me at least, the worst thing that can happen to a computer user, so any extra protection helps. You also get some flexibility in deciding which other types of web sites should be restricted.
You don't have to register to use the service and there is no software to download or install. All that's involved is a change to the networking configuration of either your computer or your router. This is a one time change, OpenDNS requires no ongoing care and feeding. Should you ever want to stop using the service, simply reverse the configuration change. I've used it for quite a while and fail to see a downside.