Below are some practical and free tools for everyday developer work. The tools are listed in alphabetical order.
abapGit is a git client for ABAP developed in ABAP.
abaplint is a linter for ABAP.
draw.io can be used online or as installable edition to create and share diagrams. You can e.g. create class diagrams.
Website to easily check your regular expression.
Website to visualize and explore OData services.
If you need a powerful screenshot tool, then use Greenshot.
IrfanView is a popular graphic viewer and a good help if you have to work with graphics in forms.
KeePass is the perfect tool to store passwords in a secure way. With the plugin KeeSAPLogon, you are able to connect to a system directly from KeePass.
This tool is helpful if you have to deal with SAPscript forms. Check this blog for more information.
If you want to start some transactions after log on to a SAP ERP system automatically, check this blog.
This tool is usefull if you work with different clients and you want to share your favorites between them. Check this blog for more information.
7-Zip is a free file archiver.