MultiAVCHD is a great free program for authoring blus. Just import your MKVs or existing blu-ray folder, then it produces a main menu/setup menu/chapter menu for you. Not as flexible as a proper authoring tool but you can still tinker a great deal with the design - use your own text, backgrounds and stuff - and get some pretty respectable results (I used it to put a fancy front-end on Harmy's 720p Empire).
Tutorial here too.