Here is a list of useful tools/api/examples:
Office 365 Developer Patterns and Practices
A series of useful tools, API and example for building applications, add-ins and provisioning for SharePoint Online and Office 365.
SharePoint CAML Query Helper for 2007, 2010, and 2013
Use this program to help build and test SharePoint CAML Queries (Collaborative Application Markup Language). Compatible with SharePoint 2007, SharePoint 2010, SharePoint 2013 and Office365, in one single convenient download containing two EXE files.
SharePoint Client Browser for SharePoint Online and SharePoint on-premise
Remote SharePoint development is getting more important. Especially with SharePoint Apps. To speed up development, find hidden lists/items/documents, discover the structure or specific artifact properties use the SharePoint Client Browser which supports SharePoint 2010, SharePoint 2013 and SharePoint Online (Office 365).
Azure Storage Explorer
Azure Storage Explorer is a useful GUI tool for inspecting and altering the data in your Windows Azure Storage storage projects including the logs of your cloud-hosted applications.