Thin profile wire joins and forks
I prefer non-insulated barrel/butt splices with heatshrink over.  It looks clean and provides a very reliable connection without solder, and at a reasonable cost.  Only inexpensive crimp tools are required.

The raw splices look like this:



The finished product looks like this (this example uses adhesive-lined heatshrink, though that is unnecessary for indoor applications)



Don't use the insulated (red/blue/yellow plastic, typically) barrel splices; the insulation often prevents a good crimp and they look awful.  



If you want the insulation built in, use crimp splices that have integral shrink insulation.