Hidden Gems
The Acorn BBC Micro is not the most talked about computer on the MiSTer forums and the "official" core is fairly limited compared to the original computer. There is an unofficial core that has a lot more features here:
- Real teletext font support
- Availability of sideways RAM
- Support for loading individual disc images
- Better application support (Acornsoft Revs works)
- Break key functional (not just ctrl-Break)
- Music 5000 synthesiser support
That last one is really cool. The Hybrid Music 5000 was part of a suite of hardware products with which you could build a sythesiser with your 8 bit micro and the effect was - for it's time and compared to the 3/4 channel sounds on the BBC - quite spectacular.
It's not a feature you will likely stumble upon; you'll need the unofficial core as linked to in this post, configure the core as a Model: BBC Master with Filesystem: DFS or MMFS & DFS(no DFS=no sideways RAM commands). (To apply these settings, reset the core)
With an AMPLE support disc image (attached) mounted, press shift-F11 (= shift-Break). You may have to do shift-F11 twice. That should get you in an M5000 software menu. Press F9 to start the jukebox and then select one of the "selections from Ample albums. Don't forget to enable sound output 
If that sounds interesting, there is a nice selection of compositions available on Stardot here.
As a bonus, with this core you'll be able to enjoy the excellent Prince of Persia and Stunt Car Racer ports by Bitshifters.
Does anyone know of other cool features of cores on the MiSTer that are a bit harder to get working but very rewarding?