I haven't been very active in the community, but I visit e621 just about every day and developed a little app for personal use. I polished it up and made it available in case anyone else might find the app useful.

The app sends notifications (with a little icon in the notification bar) when new images are posted featuring tags of interest. The app itself shows thumbnails for the most recent posts with these tags of interest. As a note, the default domain is e926.net, but this can be changed in the settings.

The app is available on the Play Store:
https://play.google.com/store/apps/details?id=com.kalahari.jackl

My background is not in programming, but it's a hobby and I want to keep working on it. Feedback is totally welcome.