Instapaper – Reading List Sync
When you launch Reading List Sync, it will sit in your menu bar and sync your Safari Reading List with your “Read Later” folder in Instapaper every hour. It doesn’t sync the read status or the current position of texts.
After you launched the app you need to open the preferences (It’s the little ‘I’ in your menu bar), and login to Instapaper. (The preferences windows has a habit of opening behind other windows, so check there, if you don’t see it at first.)
It ignores the read/unread state in Safari. This is intentional as pages get marked as read automatically (and would be removed from the Reading List after that). This is a little confusing (especially because Safari always opens the Unread view in a new window), but is better than unintentionally removing links.
The app is free, but you need a paid Instapaper subscription for it to work.