craigslist for some older speaker sets w/ big sub's...
I get "take aways" from my customers often and have an older set of Altec Lansing 2.1's that are awesome garage speakers. I also have a "take away" 2.1 from Dell that I use to rattle the floor on my desktop gamer.
The older systems had heavier sub's than most of the newer ones...
If you are looking for some real kick then stay away from desktop speakers and go with some old floor speakers. There are tons of people out there getting rid of their old large-speaker stereo systems in favor of new crap 5.1 or 7.1 setups. Go hunt Craigs list for some big 'ol speakers and amp in good condition for free or $20, and annoy your neighbors to no end... unless you are my neighbor in which case I would highly suggest a good set of headphones rather than speakers
For years I have been runing a beautiful hodge-podge system which I upgrade every time I find better parts. I am currently running some home made speakers originally built by my dad and a friend some ~50+ years ago. I purchased new subs (my sisters blew out the origionals when we were babies), and bought some good 16AWG wire (and tinned the ends... very important for cable longevity), and everything else I have put together for free. I am running a pair of 12" Pioneer subs, original 'wet' 4" midrange, and 1" titanium super dome tweeters, running with an original internal crossover which was built from scratch by my dad and his friend, and they sound beautiful! Up until now I was running an old Sansui 5000A amplifier (the version that does not fry itself lol) which was pretty clean with just a little noise from some old caps. Recently I got a mobo with optical output, and found a pioneer 515 amp. The amp absolutely sucks in surround sound (read any review and they will tell you the same), but when running stereo it is clean, full, and puts out some beautifully flat (EQ flat, which is a good thing) sound which is in a very good league of sound quality. Once you move up to optical audio there is no going back.
It's old, it's big, and it quite honestly isn't much to look at (speakers really need a new coat of paint... or entirely new boxes lol), but it sounds beautiful and smooth, and the best part is that it was largely free. Cable is cheap, and you can find similar sounding subs for free these days.
To answer you question though; Yes, Logitech is the only way to go for desktop speakers. I purchased a 2.1 set of theirs for my wife's PC and we have been pretty happy with them for what they are. They are not going to play movie quality audio any time soon, but for pandora running in the background while doing dishes and house work they are perfect for adding noise to the house.
One complaint I have with Logitech (and really it is with all cheap 2.1 systems) is that they tend to 'push' the bass too much which makes it sound rather out of purportion to the rest of the speakers in some ranges (150-250Hz). To compensate we tend to turn the sub way down, and then crank the speakers way up when we want to listen to something loud. For day in day out use it is fine though. It's just a limitation of the acoustic tricks they use to get any bass out of a tiny 6" speaker in a box, sadly all 2.1 sets (except perhaps the $120+ range speakers) suffer from similar issues.
Also, Creative makes some 1/2 decent speakers as well. I would avoid all but the most expensive altec lancing speakers, and avoid all other brands like the plague that they are!