Captain is a suite of tools related to APT.

It contains the following tools:

• an installer which is launched when you open .deb files
• an installer which is launched when you request an apt://pkgname URL

Captain replaces gdebi (which wasn't actively maintained upstream) and apturl (which wasn't actively maintained and was specific to Ubuntu).

To install a .deb file:

captain file.deb or double-click the .deb file.

To install a package from the repositories:

xdg-open apt://pkgname