Well, for simplicity and reasonable quality on a budget, and if you're prepared to hunt down some used ones, then have a look at the Logitech Z-5, or the
LaCie USB or FireWire speakers.
The Z-5s come with a remote and are powered by USB (and get their audio from the same bus), so no power supply and minimum trailing wires. If you've got a USB hub in your monitor then you can just run them from there.
Same goes for the Lacie ones, and they do a FW version too, which means a higher voltage from the bus, hence a higher wattage. No remote with those, and a very short cable. I've got a pair of the FireWire ones running off my 23in ACD and they just about reach to the end of the monitor plus 3-4in or so.
Obviously, if you want loads of bass or more volume in general, then you'll need to go for something that's not bus powered and probably a sub too.