App.net (aka ADN) was originally launched as a paid competitor to Twitter. The service then went to a freemium model, with paid members receiving invites to invite guests. However, now the service is open to anyone with an App.net Passport account.
The App.net Passport allows the creation of an account and profile, and follow other members, but does not allow posting. To post users must use a third party application. This is in keeping with App.net's focus on the app developers and moving away from Twitter's increasingly restrictive API requirements.
Free ADN users will still be subject to the free tier restrictions. Free tier accounts will receive 500mb of storage with a maximum file upload size of 10mb, and are limited to following 40 users.
App.net has stated that the open user registration is an experiment as such it may not be available for long. If you are interested in joining App.net it may be wise to get your account now while you can.
However, is it worth it? Social media networks are only as good as their community after all. What is ADN's community like? Due to ADN's focus on app development it is popular with developers who do enjoy the platform. Will it be as popular with non developers? Tell us what you think in the comments. Have you tried ADN? Will it ever rival Twitter?