An official way to display your PSN ID on various Web sites... Obviously, you will need to have a PSN account. You will also need to sign up for a PlayStation Underground account as well if you do not already have one.
Linking to the two of them together will allow you access to the PLAYSTATION Network Portable ID. The PSN portable ID "is a visual summary of your PLAYSTATION¬®Network account, including your Avatar, "About Me" message and Online ID."
There is even an HTML code provided for you to display this information wherever you please.
You can also now access your Friend List via the PSU -- in addition to the information on their portable ID, you will see their online status. If online, you can view their comment/status and what game they are currently playing. If they are offline, you can see when they were last online. At this time, you cannot manage your Friend List via the PSU.
So what are you waiting for? Link your accounts up [Register or Login to view links].
Then go to your "MY ACCOUNT" page, and on the middle left side you will see, "PLAYSTATION¬®Network Friend List", and like in my image below, I can see that Hazer is online & what game he is playing from via my PC. This is a nice feature to see who's on their PS3, by checking from via your PC first.