I've updated the historical links below using the Internet Archive for reference.
Printing a 3D model once it's "water tight" is great, but expensive: The material (and time) used to print a solid object is might greater than a hollow one with a connecting "spout" to the outside.
For this object - my original print was 22mm wide, but cost ~$10 via Shapeways.
I wanted to upscale the model to 28mm, but keep the cost from increasing drastically. With the small hollow area inside, the larger version only cost ~$11. Not a huge difference, but when printing multiple models, that space & print time can add up.
Using only free software (Meshlab and the free version of Netfabb), you can create hollowed out models for printing:
http://sites.google.com/site/3dprintfaq/workflows/hollowing-a-model
Via ZbrushCentral
It's also possible using Google Sketchup to create a simple object (a sphere, or custom drawn 3D object), then to 'flip' it's faces. This makes your inner surface. Drag / position this inside your 3D object (in the same workspace), and punch a hole in the polygons and draw a 'spout'; connecting the flipped inner shape to the surface of your main object.
Since Sketchup doesn't automatically turn spheres into polygonal objects, I had to draw the veracities for the 'spout' entrance directly on the sphere. Before, if I tried to delete the blocked area in Sketchup -- it would delete the whole inner sphere (because it was a solid object - not polygonal). This meant I needed to draw directly on the sphere with the line tool, then connect that to the outer shape.
Once that was finished, I exported the whole workspace (Sketchup .obj > to Meshlab .stl >) into Netfabb - and deleted the (now polygonal) area blocking the spout. It's a more labour intensive method than the automated Meshlab version - but necessary if the automated process won't work for your object.
2026 Update: See more updated methods online, or built into your SLA 3D printer's slicer software for hollowing models prior to printing. This, along with designing drainage holes in the prints themselves are vitally needed when printing using SLA (UV resin) to prevent islands of uncured resin from forming.