To share games downloaded from the PlayStation Store on up to 5 PS3 systems for most games and add-ons all you need to do is create a new user on your system and go to sign-up for PSN and use an existing account.
Just enter your friend's sign-in info then it should say "thank you for signing up" with their name. Then go to the PlayStation Store and click on downloads and just download what you like!
Once the download starts, you can sign off their name and it will continue to download once you sign back to your account. Enjoy!
On a related note, be sure to check PSN.PS3News.com for the latest PSN links. You can click on the column headings to sort by PSN Title, Date Added, Region and Type. More PlayStation 3 News...