Problem: you're running your favorite application when you realize that there's a feature you want that is not yet implemented. You could write this feature yourself, but perhaps there is a similar application that has already implemented it for you.

Solution: someone tells you to check out debtags, a feature of Debian-derived distributions that allows users to discover similar software packages based on the tags associated with a myriad of DEB packages. Let's utilize debtags to find an alternative for our favorite command line MP3 player, mpg123.

First, we must update the debtags database.

$ sudo debtags update

Then, we can start searching!

$ debtags related mpg123 -d 1
madplay - MPEG audio player in fixed point
mpg321 - mpg123 clone that doesn't use floating point

From the output, you can see that there appears to be a suitable alternative called mpg321. How nice! The '-d' option specifies the relevancy of the results. The default is 0, but the higher the value, the less relevant your results will be. Keep this in mind as you run your queries. The best solution I have found is to start at 0 and work your way up until your query returns some results. A value of 1 will not suffice for all package searches — we could whip up a script, but then again, I'm too lazy.

It's also possible to see the tags associated with an application.
Let's take a look at what types of tags are assigned to mpg123 that helped debtags find related software.

$ debtags tag ls mpg123
interface::commandline
role::program
sound::player
use::playing
works-with::audio
works-with-format::mp3

Now, related software substitutes should have similar tags, right?

$ debtags tag ls mpg321
interface::commandline
role::program
scope::utility
sound::player
use::playing
works-with::audio
works-with-format::mp3

This is exactly how debtags works with the '-d' option. The relevancy is determined based on the frequency of tag matches across possible alternative applications. We can also just search for software based on certain tags. Let's expand our mind and see all the sound players available:

$ debtags search sound::player
adplay - console-based OPL2 audio player
akode - (short description not available)
allegro-examples - example programs and demo tools for the Allegro library
alsaplayer - (short description not available)
alsaplayer-alsa - PCM player designed for ALSA (ALSA output module)
alsaplayer-common - PCM player designed for ALSA (common files)
alsaplayer-daemon - PCM player designed for ALSA (non-interactive version)
...[snipped]...

To veiw all of the possible tags that we can search for:

$ debtags tagcat
Facet: accessibility
Description: Accessibility Support
 Accessibility Support

Tag: accessibility::TODO
Description: Need an extra tag
 Need an extra tag
...[snipped]...

To dump the entire package-to-tag database mappings:

$ debtags cat
2vcard: implemented-in::perl, role::program, use::converting
3dchess: game::board, game::board:chess, implemented-in::c, ...
3ddesktop: game::toys, interface::3d, role::program, ...
44bsd-rdist: admin::file-distribution, interface::commandline, ...
4g8: admin::monitoring, protocol::ip, protocol::tcp, ...
6tunnel: interface::daemon, network::server, network::vpn, ...
855resolution: admin::configuring, hardware::video, ...
...[snipped]...

I encourage you to read over the man page for even more advanced debtags kung-fu. There are even options to add your own custom tags and even submit tags for packages that have incorrect or outdated information. debtags is a wonderful utility, so have fun with it! Post your comments below if you have further questions or tips :-)