For WordPress plugin developers: keep a register of when and where your plugins are activated.
Although the download stats for the WordPress repository are great, they don't actually tell you where your plugins are installed. Having an automated way of seeing who is activating your plugins - and therefore who is actually using them, not just downloading them - is fantastic to see your Open Source work actually in use.
No. The only information saved by Plugin Register is the name and version of the plugin, and the name and URL of the WordPress site it is installed on. I do not intend to ever get any persons personal information using this plugin. Registration is also manually-triggered, so no details are stored without the permission of the person who activated the plugin.