It's called LMMS (Linux MultiMedia Studio). Recently I decided to get more into learning about and making music. I'm always on the lookout for productive outlets for my OCDing and thought this could be the ticket. Also it has the added benefit of making me stop regretting that I couldn't play a note of music.
When looking for music production software on Linux, the first thing I became aware of is a lot of these programs require the JACK Audio server. So I installed it along with qjackctl (a settings GUI that also turns the daemon on and off) but got errors when I tried to start it. After a bit of googling, I found there are two separate JACKs, jackd1 and jackd2. jackd2 is not a sequel to jackd1. It's a different fork. So given I had jackd2 installed, I replaced it with jackd1 and...success! Got the server running, so now I can pass audio around between different applications if I ever need that function (moduler work environment is the applicable phrase). I don't know what the problem with jackd2 was. I got the same errors on an iBook and a Power Mac and the common advice about realtime settings didn't work. But for now I'm not gonna care.
Onto LMMS. This is probably your best bet as far as GarageBand alternatives go in that it's fairly easy to get started with but is also complex as hell just with the sheer number of options available to you. Here's the default setup with a triple oscillator synth selected:
It has a song editor for writing melodies and a beat/baseline editor for writing repetitious things like...beats and baselines. You bring up a piano roll for composing notes, and it's pretty flexible as far as copying/pasting and moving things around, etc. It also comes with plenty of instrument presets and samples for you to mess around with. And the GUI's pretty slick.
I've only found a couple of problems. If your computer's too slow, playing back songs with too many simultaneous elements may result in distorted sound, so you'd have to export to wav or ogg to hear the whole thing at once. Also, I thought at first that LMMS required JACK and I couldn't get it to start up with a JACK server running. When I stopped the server LMMS started fine, and I found in the preferences it's set to use ALSA by default. Cool, but just to see, I switched it to use JACK and got the same failure to start up even with the JACK server turned off. I had to trash the .lmmsrc.xml file just to get it to launch again. Maybe the JACK problem will be an inconvenience down the road, but as is the theme of this post, for now I'm not gonna care.
Be sure to check out LMMS's online manual beginning with the Getting Started section.
As for other programs, there's also Rosegarden. When I installed it it wanted to install Latex as a recommended package for typesetting music notation, but since I wasn't in the mood for a 1 GB download and I don't need to produce fancy musical sheets I installed Rosegarden with the --without-recommends argument. It basically does the same thing as LMMS, but in different way, so it's a matter of taste. Rosegarden requires JACK.
Also requiring JACK is Hydrogen, which is like an LMMS for drum machines. There's also Superlooper, which is about making loops presumably super. And QTractor is a newer program people are talking about.
I also tried Ardour, but I couldn't get it to launch. Maybe it's a RAM issue as the iBook I tried it on only has 256 MB (UPDATE: It was a RAM issue. I upgraded to 640 MB and it started up fine.). And Jokosher failed at the Create New Project prompt. It sounds like this PowerPC bug is still there. Oh well, Jokosher. Your screenshots looked good, but you'll have to remain a mannequin in the window.
So now I'm in the market for a midi keyboard and having to answer the existential question that has ruined larger men than me: 49 or 61 keys?