Way back in the day, my daughter and I wanted to build some simple structures using drinking straws. The typical method was to shove some paperclips into the ends, hook the paper clips together, and bit by bit assemble the structure. It was very hard, and until the whole structure was assembled, it remained frustratingly limp. I wanted a better way, and dreamed of having some tiny connectors that I could simply stick in the end of the straws and easily connect them. Then I got into ZomeTool and forget about the idea. Recently while cleaning out the house, I found the bundle of 1,000 straws I had purchased back then, which rekindled the spark of an idea I had back then. Now, armed with OpenScad skills, and a 3D printer...
This thing is finally that basic connector. But, it goes a bit beyond what I originally intended. These connectors are hollow, so you can actually use them to string bits and pieces of tubing together, and get liquid to flow between them.
This is very similar to the type of connector found in drip irrigation systems, like the ones found at Home Depot. They are called "barbed connector" there. You could probably also make the "speared" kind to dig into the thicker pipes.
Since these cost about $0.05 to print, and the store bought ones are more like $5, you might realize some amount of savings, if you're into creating these things.
I use straight angles here, so if you really were trying to form some structures, you might want to do the extra math to deal with angular defect. Or you could look at the library here: http://www.thingiverse.com/thing:9560 and dig out some of the math.
The .stl files here are meant to fit within the 6.25mm inner diameter drinking straws that I have here at home. You can simply change the OD value at the top of the file to mate with whatever 'tubing' you so happen to have. It will work great with vinyl tubing. A little dab of glue, and a tight fit, will make for a nice tight permanent structure.
Another fun thing might be to run some elwire through the tubes for a fancy display. Who knows. The possibilities are endless!
The OpenScad is included so you can make your own. For the elbows, just specify an angle. For the others, just specify how many connectors you want.
Added blog entry here: http://williamaadams.wordpress.com/2011/09/05/useful-household-items-drip-irrigation-connectors/