I use Sketchup. There are dozens of free software for both inside and outside the house. You just need to find one that suits your needs best. Sometimes the learning curve may be a bit high, but when you reach the summit nothing is beyond you. - EricL
I use a free program called TinkerCad. It takes about an hour to start getting the hang of it, but it's great for putting simple designs together. SketchUp is wonderful, and my colleague @EricL is an absolute whizz at it, but it has a steeper learning curve. Also check out the Gardenia Garden Planner; I find it quite useful when laying out garden designs. - MitchellMc
Pen and paper for me initially, then I use a program called Smartdraw to put everything together. This program has heaps of templates and you can even insert furniture to see how everything lays out. This does cost, however I am pretty sure you can download a trial version. Can be a little tricky for first time users but has a very good user guide. I also use this for electrical wiring diagrams and landscaping layouts. - darylhewston
I believe a few of our members use a free tool called SketchUp for a variety of 3D modelling projects. For specific applications like landscaping, you might like to search for a tool designed for that purpose. Personally, I've been using another free software called Tinkercad. It's straightforward to start with and is a great stepping stone before moving onto other more complex CAD programs.
Recently I built a sandpit with a foldable cover, and putting a 3D model together really assisted me in understanding how exactly the folding mechanism worked before getting started. I find it very easy to use compared to other 3D modelling programs.
I mainly use Autodesk's Fusion 360. It's simple to use once you get used to the concept. But it's ridiculously powerful and can do drawings for 3D printing and CNC as well. It has a hobby user licence so you can use this powerful software for free. Whenever I'm designing toys or other stuff then this is my go-to app. It blows SketchUp out of the water in terms of power but there are more resources for SketchUp.
However, for designing floorplans, etc, I'm using an application called Sweet Home 3D. It allows you to draw a 2D layout of your floor plan and it will automatically generate a 3D model of that plan. I'm currently using it to make plans for our house so that I can write a document for the council with regards to what we want to do. - woodenwookie
For iPhone: Scribble It! For Android (& iPhone) You Doodle. For a PC, Irfanview will do almost everything you want to with pictures, including allow you draw over them. - TedBear