The boot sector is only 512 bytes, but if you're using Asm that's actually more than enough for some nontrivial functionality. Here's a few of many examples of "what fits in a boot sector" that has appeared on HN:

https://news.ycombinator.com/item?id=21951241

https://news.ycombinator.com/item?id=20569438

https://news.ycombinator.com/item?id=25531677