Here's a collection of resources that the community has found useful in getting started with the x16. I'll try to keep the list up to date as the x16 evolves and as people create more resources. I am in no way affiliated with the x16 team, just someone watching from the sidelines.
Generic resources for getting started with basic. These mostly focus on C64 BASIC which is 99% compatible with the x16. Note that due to differences in memory layout, PEEKs and POKEs for the C64 are (for the most part) incompatible with the x16.
As the C64 has been used as the basis of the x16, there are many similarities between the two systems. Here is a collection of general C64 resources that may be useful for the x16. Note, there are many incompatible details between the two systems, so please don't take details in this section too literally.
MON - Built into the x16 firmware. This is a modified variant of the Final Cartridge III monitor. It will be replaced in an up and coming release with something better, but it's a good start for poking around in memory to see how things work.
The hardcore option, for those that don't need labels or macros. Baked into the x16, so its convenient for debugging on the go. The current implementation is a derivative of Final Cartridge 3, however there will be a new MONitor coming to x16 soon.
Part of the cc65 bundle. More features than 64tass, and a somewhat nicer syntax. More difficult to get started with, you've gotta deal with "linking" to position your code at the correct locations in memory. Allows mixing assembly and C.
A C compiler suite targeting the 65xx family of microprocessors. The latest source version of the compiler (must be pulled from github) now officially supports the x16. There is example code available in the x16-demos repository.
A language designed specifically for use with the 6502/6510 series of microprocessors. Supposedly more efficient than other high level languages, such as C and Pascal, due to targeting 65XX specifically.
A Pascal like language and IDE suite targeted at a number of 6502 and 68000 platforms. Experimental x16 support has been added to the latest development builds, and will be available from release 0.9 onward (should be available November 2019).
An 8-bit IDE plugin for Eclipse. Supports a range of different assemblers and platforms. No explicit support for x16, but should be fairly easy to get going with minimal tweaking (choosing C64 as your platform should mostly do the trick).
A Build tool that verifies and converts source PNG files into VERA-compatible graphics formats. Supports all Vera types such as text, tileset, sprites and bitmaps, and outputs data files ready for use with Assemblers, cc65 header files, .BIN files, and even BASIC DATA statements. Also contains a series of samples and tutorials useful for VERA newcomers.
Great little freeware utility for composing tracks targeting a number of old sound chips including the YM2151, C64 SID, YM2612 (Sega machines) and others. The sound chip for the x16 has not yet officially been chosen, but it appears likely that it will be the YM2151.
A fantastic series of guides for getting started with the x16 and VERA in assembly. Looks to have not yet been updated for VERA 0.8, so you'll need to refer back to the VERA docs or stick with x16emu r30.