I’ve also taken to using OSH Park. Especially when designing boards for projects with my maker group. I don’t have purchasing ability with them (I don’t hold one of the purchasing cards). So I design the board in KiCAD, upload the .kicad_pcb file, and then send the order link to our purchaser with the desired quantity. Since all the projects that I’ve done so far we’ve been targeting as OSHW, it isn’t a big deal that as soon as I generate a shared order link is can be publicly found on OSH Park’s website. YMMV
I used PCBCart in the past. They have a nice online pricing tool and they also are pretty good at spotting any design issues that one might have in the board design. They have a large capability spectrum. They are not ultra cheap but they are reliable, encourage good pcb design practices and the PCB quality is on par with the price.
For prototypes I use OSHPark, they have great balance between price/speed/reliability. The PCB quality is extraordinary, much better than PCBCart or any other low cost prototyping service I have tried so far. (maybe Eurocircuits is comparable, but the prices usually are much higher for similar lead time, and I am now located in the US so they are not really an option anyways)
Recently I also make my production runs through OSHPark too. Unless I need some crazy 8layer board with blind via I will likely continue using them for all my standard spec boards. Additionaly they are a very communicative and helpful bunch. If you have some special requirement email@example.com is very quick at doing what they can to help out.
If you looking for boards + assembly, you may want to check out MacroFab. I just ran a small board fully assembled with them as a test. There system auto-quotes the turnkey assembly. Their automated system did swap a resistor to an alternate vendor with the incorrect value (double check everything before pulling the trigger!), but customer service was very fast to get the issue fixed.
For small quantity PCB prototyping, especially when you’re located in USA, I would recommend OSHPark, fantastic purple boards, perhaps the best price you can get within US when taking the quick shipping into consideration.
When comes to mass boards production, I recommend PCBCart. It specializes in fabricating circuit boards for high end products, quality standard is much higher than prototype boards. Its balance between price/speed/reliability is among the best Chinese manufacturers. They have components sourcing and circuit boards assembly service.