The "home" end of the 3D printer market has 2 general types. The DIY/open source type and the "off the shelf" type. I've selected an off-the-shelf UP! Plus printer. This type was reviewed in Silicon Chip magazine -
http://www.siliconchip.com.au/cms/A_112546/article.htmlThe price has since plummeted from that quoted in the article. I paid under $1900 for mine. The advantage of the off the shelf types is that they are ready to go pretty much out of the box, whereas the DIY jobbies can take significant time to assemble and tune for quality printing. A guy at work is building one and I've printed him all the plastic parts which are available for download from the internet.
They are really most useful for rapid prototyping rather than final product. However depending on the final use you can make final products successfully. Not sure I'd use it for making rudder pins. Hobie put a lot of effort into getting the material right for the job and I'd rather trust their pins than untested printed ones. I'd certainly give noncritical items a go though.
The ABS plastic comes as filament on spools and is available in a range of colours. It's supposed to be relatively UV stable. You can also use PLA plastic which is a renewable resource from maize or corn starch. It dissolves in water however so may not really work well on a kayak...
See here for a 3D printed ball bearing (skip middle part of video)-
[youtube]http://www.youtube.com/watch?v=GOEbZ98J1Ks[/youtube]
Many existing objects can be downloaded and printed from here-
http://www.thingiverse.com/Custom objects need to be designed in a 3D CAD package of some type. Google Sketchup is a free one that will work. The file needs to be saved as an STL file for import into the UP! printer software. Learning the CAD software can be a significant learning curve if you want to make your own stuff.
Last thing worth mentioning is that larger objects printed in high quality can take a while to print. Longest print I've done was 10 hours for custom extension tube for an optical gizmo a guy at work was building. Think he's looking at getting his own printer now....