you could limit the "require valid-user" to some methods only by putting it into a <limit> section:
<Limit POST>
  Require valid-user
</Limit>

[download]
This should prevent unauthenticated users from posting (if you make sure posting is only doable via POST and not via GET of course ;)
Or you could simply create a guest account (e.g. user guest with pw guest).

Update see http://httpd.apache.org/docs-2.0/mod/core.html#limit for more details
Update 2: mod_auth_anon could be of particular interrest for you

In reply to Re: [OT] apache authentication default user? by Golo
in thread [OT] apache authentication default user? by Jaap

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