To my chagrin, I started working on a favorite posts plugin that IS for registered users only.
This will eventually include a profile page for members that allows for quick viewing of favorites.
Progress on this, is, well progressing. We have more or less, completed the front end, minus some tweaking of the actual display, and I'm hoping to start work on the profile page output later today.
This is a pretty large endeavor for me, and I'm hoping to solicit some beta testers (even now, for the uncompleted stuff).
Im working on this with a vanilla install of 2.3.2 with PHP4, so I am looking for someone that is sure that they are using PHP5. Your theme makes no difference.
Understand that this is not complete yet and any tester(s) will only be able to set and unset the favorites via the actual links -- not via the profile page. (Yet)
If you are interested, click on my linked name here on the forums, follow the link to my site and contact me via the contact form on my site.