Had a interesting issue, our WSS 3.0 (SharePoint) internal site wasn't allowing users to upload/edit or delete files in the document library. On closer inspection everything was correct with the library permissions. Then I noticed the "View All Site Content" hyperlink was also missing. So what had happened to SharePoint? Well it seemed the morning's backup had failed and SharePoint had placed the site in a read only state (I presume to protect users adding more content when we didn't have a good previous backup file).
Open SharePoint 3.0 Central Administration
Select "Application Management tab"
Under "SharePoint Site Management" section - select "Site collection quotas and locks" hyperlink
"Site Lock Information" section is something other than "Not Locked" selected (ours was set to Read-only (blocks additions, updates, and deletions).
Just tick "Not locked" and your site should now work correctly.