I'm not going to bore you with the history of the Billy bookcase, the Wikipedia entry can. What the history page won't tell you is that it's also a pun, as the Swedish word "billig" (usually pronounced the same as Billy) means "cheap" or "affordable" in English.
They're so common and affordable, that they're almost hard to give away for free when you're bored with them. There are many different thrifty or reuse options though (I butchered one I got used for free for my custom printer and NAS station).
This is not intended to be a curated "7 ways to trAnsForM y0uR bIlLY!", just some inspirational examples. Google IKEA Billy hack and you'll get thousands of results. Just stay away from Pinterest, a lot of the examples there suffer from linkrot or link to cancerous ad blasted pages.
Built in bookcase
The below is just one example, but it's nicely done with integrated wall plugs, light switches and fancy light fixtures.
Source: The Sommer Home
Horisontal TV bench
Source: Dalia E. Aly [Instagram]
Switch out the doors and give it a new lick of paint
Source: Lara Bezzina
Thrift several Billys, use one for scrap material for gabled roof and make a doll house or two
Source: Tamara Central
Fancy shoe shelving in a walk-in wardrobe
Source: Connie and Luna - how to organise your shoe collection
Thrifty not-so-fancy shoe shelving in hall closet
Source: My own messy hall closet. I thrifted some really old Billy's that were 75 cm wide, which fit perfectly inside my 80 cm hall closet, chopped the top part off of one of them and voilá - shoe shelving for free. More info in the linked post.
Enough of my ramblings, this is starting to look like Pinterest (sorry). Go get some thrifted or freecycled Billys, go forth and experiment.
Thank you, I am glad that you liked it.