Alfrescos vast network of partners have developed many add-ons to Alfresco. Thes add-ons are made available either as open source licensed or commercially licensed.
You could find all the available add-ons at the alfreso-add-ons site
We at Redpill-Linpro have also developed and released Alfresco modules not classified as add-ons that we have published at github.
For the following modules that we have been writing about in past issues of the ECM Newsletters there is more information documented
Alfresco Numbering module
Alfresco Numbering module automagically generates number series useful for documentID or other purposes in Alfresco, The number series generated are customizable to fit customer requirements. Development of this module is sponsored by Alingsås Kommun.
Alfresco System Messages module
gives system administrators a way of notifying Alfresco users with information such as planned system maintenance or general information.
Read about the details of these modules in this blogpost