*Disclaimer Intel Chromebooks Only*
YOU CAN INSTALL THE FULL VERSION OF UBUNTU NATIVELY ON A CHROMEBOOK AND REMOVE CHROME WITHOUT USING CHRUBUNTU
I'm seeing a lot of people, in questions like these, answer saying that you can't remove ChromeOS from a Chromebook, install straight Ubuntu due to Chrome's custom BIOS not allowing it, or you have to install it using Chrubuntu which is deprecated and dangerous to install on newer machines.
I want to clear that up and say you can.
I flashed SeaBIOS on my Chromebook and installed Ubuntu 17.04 from a flash drive, however, I think that SeaBIOS also allows you to install an OS from Jeltka (I'm not sure what that is and I didn't use it; I already had the flash drive made).
SeaBIOS boots from your local hard drive by default but, as it's loading, you can tap ESC and boot from any device listed. This lets you run a native OS as well as any other OS that is compatible with your hardware from a USB.
To replace Chrome's custom ROM and BIOS and install different OSs, you have to first put your Chromebook into
To invoke Recovery mode, at login screen, you hold down the ESC and F3 keys and tap the Power button.
To enter Dev-mode you first invoke Recovery, and at the Recovery screen press CTRL + D (there's no prompt - you have to know to do it). It will ask you to confirm, then reboot into dev-mode.
Dev-mode works the same as always: It will show the scary boot screen and you need to press CTRL + D or wait 30 seconds to continue booting.
After this, you will need to bridge the write protect jumper with your chromebook off and preferably with the battery out!
This is where the write protect jumper is on a Parrot Chromebook (Acer C710); it will probably be located in a different place on your Chromebook if it's a different model.
That little white square at the end of the pink tab is the jumper (it's under a black tab that you'll have to peel off); you can probably bridge it with a bit of solder, if you want it permanent. I used a piece of aluminium foil.
Once you have done this, you'll need to reboot your Chromebook, open your browser, then press CTRL+ALT+T to open the shell.
MAKE SURE YOU READ LEWIS'S SUPPORTED MODELS MATRIX
Once you have read the developer's supported models matrix, you can run the following command in the Crosh shell as a normal user not root
cd;bash <(curl https://johnlewis.ie/flash_cb_fw.sh)
Finally, follow the on-screen instructions according to your model and what the matrix says it supports.
Once you have done all of this, you will be able to install an OS from USB to your SSD, run an OS from live USB, or, now, again, I'm not sure how this works so you should do some research (I couldn't find much when I was looking), you can boot from payload which, I think installs an operating system of your choice from the ones available.
Live Flash Drive
If you want to boot from a USB, here are instructions on how to boot from a USB and install the operating system on the USB so you can use it on any computer.
Making an Ubuntu Installer Flash Drive
Here's how to make the Ubuntu installer with Windows.
Here's how to do it with OSX.
Here's how to do it with Ubuntu if you have it on another computer or something.
Here is straight Ubuntu. This is the full version with all the default packages, etc. Ubuntu 16.04, the Long Term Support release, uses the UNity desktop environment and Ubuntu 17.10 uses the GNOME Desktop Environment. I recommend using one of these or GalliumOS, discussed later, unless you speak Chinese, in which case, Ubuntu Kylin would probably be the better option.
Personally, this is my opinion, I would only be concerned with flavours, to start off with, if you have a really old computer; then, Lubuntu might be the better option. Flavours are really fun to test out, see which ones suit you more (or less), etc. Personally, I prefer Ubuntu GNOME but it does use more RAM (in my experience) than Ubuntu with Unity (default). Just make sure you read the descriptions and look at screenshots for all of them before blindly choosing one; It's annoying to have to go back and reinstall stuff.
EDIT: 15-10-17: I have just found/tried/installed GalliumOS and it runs even better than Lubuntu does. It also has full support for ChromeOS devices so the touch pad issue below doesn't apply. It is also further optimised for ChromeOS devices because it is based on Xubuntu. I am really enjoying it because it's fast, beautiful, and has full hardware support, unlike any other Ubuntu flavour/derivative I've found. In addition to the other optimisations, the keyboard is entirely remapped so all of your brightness, volume, media control keys, etc. function properly instead of just sending the FX signal.
On an Acer C710 (Parrot Chromebook), the touch pad won't work (I'm working on installing the Ubuntu-modified ChromeOS touch pad drivers) and neither will the brightness keys, volume, refresh button, etc. I'm not sure what the issue is there yet, but I am working on it and, if you do what I've said here, and want the drivers/keyboard-fix-stuff, ping me and I'll edit those into the answer once I've figured it all out.
Also, you might want to backup any important files that you have to Google Drive or something; when you install SeaBIOS, all of your files will be erased and unrecoverable.
As I've only done this on a C710 (Parrot) Chromebook, I cannot attest to what will/won't work on other models but you run the same risk putting Ubuntu on a Chromebook as you do with putting Ubuntu on a different computer; some features (WiFi, Bluetooth, Touchscreen, Touch pad, etc.) may not work due to missing drivers. All you'd have to do is find those and install them or modify the ones you currently have;
That's the beauty of GNU
I have found the drivers! (sort of)
GalliumOS (referenced above for its compatibility) comes with out-of-the-box touch pad support.