• Last.fm logs the music I listen to on the computer via a plug-in
• del.icio.us lets me store my links on-line

Both of these provide RSS feeds that I can publish on my site. They also support the use of tags to categorise items in different ways, but I've not used that for music yet. Not only is everyone's data public, but it can be used to recommend other things you might like.

The latest variation is Reader2 where you can record books you are reading or have read. It's fairly basic at the moment, but evolving. Some of the feeds do not seem to work properly and recommendations seem to be limited to showing other users who have some of the same books. I'd like to see more options for status. I'd like to be able to specify more clearly how good I think a book is. It doesn't yet allow linking to friends.

Actually I already have something similar available via Amazon. That does a fairly good job of recommending books, music and films based on what I say I own and how I rated it. This is valuable information to them to encourage people to buy more stuff. I just prefer to see more open implementations of the technology.

Ultimately I'd like to write my own tools for all this, but it's just easier to use something other people have done. In the past you might have used software on your PC to log personal collections, but I prefer the on-line options as I can use them from anywhere. The usability of web pages is improving and making software redundant. For me the limitation is how much of my personal information I trust others to look after.