Clementine

I like Clementine because it has a great interface with which you easily get along with :)
Although i only use it for the mp3s stocked on my computer, you can pair it with Soundcloud, Spotify and such other online music services, podcast websites or even online storage websites such as Dropbox and Google Drive. It's great for creating playlists that mixes music from artists only available to you on specific platforms!

One button i use very often since i installed Clementine is the "Stop playing after this track" button. You can also queue tracks so that, within a playlist, you can create your little temporary playlist for this particular session. It's great.
Talking of playlists, it does a great job at letting you create any playlist you want, name it, export it, import it. Anf if you're too lazy to create your own, the music player has dynamic ones already created for you.

It also satisfies my collector's desperate need for tags and classification in my music librairy, as this music player allows you to quickly order your collection by any information available. It even has a tag editor integrated, which is quite simple to use.

On top of that, Clementine offers many other tools you might need the use at some point such as, trippy track visualizations, rain sounds, imported lyrics, author, album and track imported descriptions and an equalizer.

So yeah, i think it's a great software :)

EDIT: i now use Strawberry instead, which is the maintained and open-source equivalent to Clementine :) but my review here still stands!

This moderately lightweight app has everything you would need from a desktop music player without all the hassle of extreme tweaking (i.e. Foobar2000). It integrates my Grooveshark.com library very well, along with my music collection on my iPod.

It's great on Windows, but the last time I used this program on Linux all it did was crash. It would crash every 30 to 90 minutes, and if I closed it manually or let it crash, the system would bog down due to memory leaks and memory that the program wouldn't let go of after it closed, and sometimes it wouldn't let go of pulseaudio or the alsa device, so if I needed to hook my audio interface into my DAW via JACK later, I could not; the only fix was to logout and log back in or entirely reboot the system. I encountered this with Realtek and VIA based onboard audio, as well as with Zoom R8 and Behringer U-Phoria UMC404 HD USB professional audio interfaces, on systems with AMD FX-6300 (AsRock 970 VIA or Zoom R8), Intel i5-6402p (MSI 110H VIA or Zoom R8 or UMC404), and AMD Athlon 200GE/Ryzen 5 3600 (socket AM4, 2 different AsRock B450 motherboards with different audio codec chipsets from Realtek, both USB audio devices). Also, on my AMD FX system, sometimes, I would get hardlocks if I tried to exit the program while music was still playing, requiring me to Alt + SysReq + R-E-I-S-U-B (the Linux equivalent of Ctrl + Alt + Del, but a bit more extreme - it's better than just unplugging the system). I encountered this hardlock on Ubuntu 16.04, Linux Mint 18.x, Zorin OS 14, as well as concurrent releases from Debian, Fedora, Manjaro, and PCLinuxOS at the time. The program works fine, however, on Windows 7 and Windows 8/8.1/10, though, as my ex had it on her Asus Transformer Book and her desktop, at my behest, as a "just in case" backup for when iTunes wanted to be derptastic special (which would happen from time to time), and she needed to backup her iPhone or otherwise access it's contents in a hurry. This Linux behavior, however, is unacceptable. Being as I use both Windows and Linux on different machines, I cannot recommend this program. If you only use Windows, glhf, you'll probably love it. If you like having the same software on multiple platforms for continuity, or only use Linux, AVOID LIKE THE PLAGUE.

[Edited by needsLITHIUM, July 05]

Simple and extremely lightweight
Once well configured it's doing all you could want for a daily use in background

It is better than Rhythmbox. Rhythmbox has no customisation whatsoever. This one has tons of them.
It is also better than Audacious because most of the default customisation is already good to work with. Tweaking them is also easy here.

Everything I need for setlists when drumming. Very easy to use. I love the tabs, how easy it is to rename them, and how fast it is to switch between them during song play. Equalizer, and other sound effects, all in a small package.