Salesforce is very well supported on Zapier. You can Trigger off the creation of any object inside of Salesforce, including Custom Objects which might have been installed by another 3rd party application. Likewise, you can create any object inside of Salesforce as an Action.
If the object type you want to Trigger off of, or create, isn't visible inside the dropdown menu, choose the "Custom Object".
Later on inside the Zap editor, you'll be given the chance to decide which Custom Object you want to Trigger off of, or create.
Note: You can generally choose any object from the list for triggering. However, on the Action side, certain objects may not allow creation. If after choosing a Custom Object type, you can see no fields (and just see a "Continue" button) this implies there are no fields to create and you'll need to either choose a different Custom Object or an administrator will need to modify the object definition inside Salesforce, first. Here is an example of a Custom Object that does not allow creation (it has no fields):
