Brightkite is a social network with a twist. The emphasis is on discovering people located near you.
People located near you are not necessarily people who live near you. Most people wouldn't want to give their home address to a bunch of strangers, and brightkite.com doesn't expect you to. Your IP information is concealed. You can give as little or as much information about your address as you choose - for instance, just 'check in' with the town name.
In addition, you are encouraged to check in from wherever you are currently - and especially wherever you wouldn't mind meeting other brightkite users. Check in on your laptop where ever wireless internet access is available. Check in from a local public library if they have computers available to the public.
The system has great potential for meeting people to play chess with, trading used paperbacks you've already read without paying postal fees or making multiple trips to the post office, meeting wargamers or people who like to argue about politics. Right now the site is still in beta, and even in fairly urban areas only three or four people per month may check in within ten miles of you.
Oh yes, casual sex. I'm extremely shy and happily married, but knock yourselves out.