Since we're all into this OpenID thing, I think it would be really useful (and quite easy) to implement a friend invitation feature that grabs a list of friends from one's webmail contact list with an option to choose who to invite to SO, SF or SU. Something in the line of Facebook's friend invitation and recommendation feature.
How to invite friends?
- Find a great question you think they might be able to answer well. Send them a link, invite them to answer.
- Send them links to your profile
- Put your SO/SF Flair link on your blog
- Find a 'question of the day' to write on your whiteboard at work; change it frequently with a new number.
- Put a link to your SO/SF profile in your email signature, forums signature, etc.
- Put the top 5 voted questions RSS feed on your blog