I thought this video showing a Review of a Harbor Freight Kit that welds plastic was interesting. Are there other type of kits out there?
You can actually get a liquid plastic welding compound. That's primarily intended for things like plasti-card and similar, but will work with any polystyrene type plastics (like model plane kits etc...). You simply paint it onto the parts you want to join and they will slightly melt and then as it cures it re-forms as a solid joint. Basically like a more powerful polystyrene cement.
As for other types of plastic, well that's a different matter, I'm not really aware of anything, but there again I've not looked too deeply, would be handy for me though what with all the multi-part models I print. Although at the moment I use either an epoxy resin or polystyrene cement (the latter works on smaller parts or if you have lots of time and can brace the parts together that you want to join).
Happily collecting things all my life... 
That looks useful for plastics that are difficult to glue, like polyethylene, Delrin and nylon.
The video makes it look so easy. Wish he showed what the other side looks like.
