HP's short-lived Touchpad had nice specs - quick CPU and large tablet screen. It also had the WebOS operating system which suffered from lack of developer support and a small app store.
A few years ago I installed a CyanogenMod Android on the HP Touchpad. It was a tricky process. I recently refreshed the Touchpad with CyanogenMod 11, and the install was simple due to a tablet app from the folks at xda-developers.com which guides installing the pieces. The install still requires a PC with some communication software.
From a user perspective, it is straight-forward to install a recent cyangenMod on the HP Touchpad.
1) install novacom drivers on PC to communicate with tablet
2) download three files (ROM, Gapps and Recovery)
3) run TPToolbox and choose "Complete Data Reset"
Download novacom drivers from: http://downloads.codefi.re/jcsullins/webos/novacom
Choose a Recovery (CWM, Philz CWM, or TWRP) like this http://forum.xda-developers.com/hp-touchpad/help/rom-guide-how-to-install-android-4-4-x-t2912820
Choose a Gapps like this http://fs1.d-h.st/download/00114/NTU/gapps-kk-20140105.zip
Touchpad Toolbox (tptoolbox) is a user-friendly system for the low-level management of the HP Touchpad. Tptoolbox makes the refresh simple. Get TPToolbox from http://downloads.codefi.re/jcsullins/cmtouchpad/tptoolbox/TPToolbox-2015-01-08-v42.zip
a) Install novacom on the PC.
b) Fully charge the Touchpad.
c) Put the Touchpad into webOSrecovery mode and connect via USB cable to PC. With the Touchpad off, hold VolumeUp and hold Power until you see the large USB symbol.
Let the PC load the communication drivers. If there is a problem, on the PC ensure USB is set as a media device (MTP) and not as ADB.
d) On the PC, extract the contents of the TPToolbox zip file. Navigate to the tptoolbox_v42 directory. Run the batch file named tptb_v42_[OS].bat
The tablet will disconnect, reconnect, and there will be much scrolling text on the tablet. Touchpad Toolbox application will appear on the tablet. Use the volume buttons to navigate tptoolbox.
e) Optionally reflash the A6 firmware to reset battery management.
f) For the most reliable install and to remove webOS and the space it was using, do a "Complete Data Reset". There are three confirmation prompts before it wipes data.
g) Choose "Install Android". This will mount a PC drive containing a folder named "ttinstall".
From the PC, copy the ROM, Recovery, and GAapps zip files to the ttinstall directory.
When the three files are copied on the PC to the new ttinstall folder, go to the tablet and choose "Unmount Media Volume".
The files will be verified on the tablet. Select YES to approve the sizings.
Wait for a reboot. The tablet reboots and the update is installed.
h) Tablet restarts and boots the updated CyanogenMod. Choose language, setup wifi, setup Google apps, and other settings.