RefreshCache is a snippet for MODX Revolution that refreshes all cacheable resources and the chunks and snippets they contain -- whether they need it or not. It is slow and inelegant, but when it's finished, site visitors will not have to wait so long for pages to load.
Notify sends email updates to users on your site and (optionally) posts to Twitter and Facebook. The users can be selected by User Group, and/or by tags specifying user interests. The updates are manual, rather than automatic, so Notify only sends them when you tell it to.
MessageManager allows user to read, create, and reply to MODX Manager messages in the front-end of your site. Messages can be sent to individual users, user groups, or all users. You can let users opt out of the message system based on User Group Membership.