How to panelize gerber files?

I use the open source utility GerbMerge, it gets the job done nicely. The utility can be found here

Edit: looks like there is another version here that could be more up-to-date:

I use ThisIsNotRocketScience Gerber Panelizer software. It works very well, is open-source, so I can fix edge case bugs myself if necessary, have made and ordered at least 5 different panels with it, without any problems. Here's a tutorial on this software I wrote on this some time ago.