There's also GardenMateApp.com online garden design app with 3D preview. It seems quite straight forward...
I used smallblueprinter to plan my new garden. It takes a little while to get the hang of, but If you download the free 14 day trail when you know you've got a few spare days ahead then you can do it for free, as I did..
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I've just realised that I may have breached the terms of this forum, sorry everyone. To remove my original post I've replaced it with this.
I'd add my vote for Sketchup.
It takes a little while to learn, but there are loads of great videos and guides, plus ready made "components" that other people have built that you can import.