For general Linux Desktop recommendations go to [EmulationStation DE: User Guide](../../wiki-rtd/docs/wiki_devices/linux_desktop/linux-software.md) as many of them also work on the Steam Deck.
Is a great piece of software that allows custom plugins inside Steam Deck's gamemode. It can open up more advanced features then the standard Steam Deck experience offers. But it's a bit more technical to install, please follow the guide on their github.
PowerTools let's you do advanced tweaking of the Steam Decks hardware. This can greatly improve performance on certain games in the PS2,GC and above generations. But it requires you to find the knowledge of what setting works best for each game and/or emulator.