If it's in the ribbon of the forms or view of a list/document library, then you can add it though SharePoint Designer by opening the list and then use the Custom Action button in the new section of the ribbon.
Initially it may look as it's very limited what you can do in the custom action, but one of the options is to navigate to an url (with parameters like Selected Item) this can either be the url to a page with does a lot using client side object model and the supplied parameters or it can be javascript directly in the link. Another option is that the ribbon button can initiate a workflow on the selected item.
If "without Visual Studio" means "without Farm Solutions" but "Sandboxed solutions" are allowed then that's an option but of cause that would not really be "without Visual Studio".
If "without Visual Studio" means we'll not pay for that tool, then using CloudShare might be an option.