Many mobile apps especially don't work with Zapier yet—do you have any that you found a workaround to get to work with Zapier?
The most handy workaround I've used is email. Many apps—especially to-do and project management apps, but also reader apps like Pocket, notes apps like Evernote, bookmark tools like Pinboard, and more—let you email in new items. If your app doesn't have a Zapier integration, check to see if it has an option to email in new data.
If so, then you can use Zapier's email or Gmail action to send an email to the app's email address for a hacky way to connect apps that don't integrate with Zapier yet.