Following on from http://buytaert.net/comment-notifications-via-email
Here's a quick review table. Please provide advise on how to improve it. I only listed characteristics for which they are different - e.g. all have MySQL support so why bother listing it?
[Update - May 13, 2009] Comment Subscribe has been merged back with comment notify. The users of comment_subscribe have been provided an update that will migrate them seamlessly to comment_notify - #361449. Also see this discussion (#224782).
| Metric | comment_notify | watcher | notify / notify by views | subscriptions | notifications |
| Anonymous | Yes | Yes (D6) | No | No | Yes (v4.x) |
| PostgreSQL support? | Yes | Yes (D6) | Yes(D6) | Yes? | Yes? |
| "Lightweight" | Yes | Yes | Yes | No | No |
| Actively supported | Yes | Yes | Yes | Yes | Yes |
| Subscribe without comment | Not Yet | Yes | Yes | Yes | Yes |
| AJAX-subscription link | No | Yes | N/A | No | No |
| Extensible | Not much. It's not a goal. 7.x lets you add tokens. | Not relevant. | Not Yet | Yes | Yes |
| what can extensions do: | ... | ... | ... | ... | ... |
| Supports message templates with tokens | Yes (kinda) | Yes | Yes (Notify by Views) | Yes (with String Override or Mail Editor) | Yes |
| Users can share list of subscribed nodes | No | Yes | ? | ? | ? |
| Scalable queue-based email dispatcher | Yes (via Queuemail) | Yes | ? | ? | Yes |
| Notify on node update/change | No | Yes | Not Yet | Yes (updates and/or comments) | Yes |
| Integration with Node comments | Yes | Not yet | ? | ? | ? |
| Digest email? | No | No | ? | Yes | Yes |
| Organic Groups? | No. | No. | ... | ... | Strong integration. |
| How actively updated? | Stable module, bugs fixed every few weeks. | Stable and actively maintained. | ... | ... | ... |
| Brief description | Provides a checkbox during comment submission allowing the commenter to get e-mails about a node they comment on (for either replies to their comment or all comments on the node). | Allows any user (including anonymous) to easily subscribe to comments and updates to a node. | Sends mails to authorized users with notifications of new nodes and/or comments. Notify by Views adds the capability to use Views to designate which nodes should be sent to users. | A generalized subscription and message sending framework. | A generalized subscription and message sending framework. |
| WHY this module over others? | Lightweight module that helps keep people updated about replies to their content. | Very user-friendly and simple to use, allows anonymous users to subscribe to nodes, allows email on updates and new comments. | ... | ... | ... |
| Download statistics Jun 13th | fast expansion, now slower to 7000 | stable progress to 1000 | expanding, unstable consolidated bit to 4300 | stable progress to 5500 | strong progress with 25000 short peak, now 12000 |
| Drupal 7? | Dev release available. D7CX pledged. | ... | ... | ... | ... |
- All of these modules provide support for 5.x and 6.x versions of Drupal core.
- None of these modules provide support for rss feeds for comments (though Views can).