Developer's note:

Half-Byte Loader (HBL) is an open source Homebrew Loader for the Sony PSP. It allows to play fan-made games on any PSP up to firmware 6.20. HBL allows to run emulators for the Game Boy, Game Boy Advance, Nintendo 64, Sega Megadrive, NES, Super NES... and many other applications. HBL is currently the ONLY way to play homebrew games on any PSP that has a firmware 5.50, 5.55, 5.70, 6.00, 6.10, 6.20, including on the PSP Go. HBL Currently does NOT work on firmwares 6.30 and 6.31, but this is under development.

Changelog:

• First, a graphical glitch has been fixed. Because of that, Homebrews such as PSP Rhythm were technically unplayable (@Muffinhead: you owe me ;) ). Others such as FCEUltra or Doom had minor graphical issues because of this.
• Additionally, I added a shortcut to quit homebrews that don't have an "exit" button. Yes, the "home" button is still broken, but now you can exit most games by pressing select+start for a few seconds. (Note that some homebrews such as gpsp still crash upon exiting, this is unrelated). The "select+start" combination can be changed in the hbl config file, or even customized for each homebrew (as usual, by putting a hbl_config.txt file in the homebrew's folder)
• The distrib version now ships with wMenu 0.3, which allows you to navigate in subfolders, so you can now split your homebrews in several folders if you use wMenu
• Other minor bugs were fixed, which shouldn't make any visible difference (check the changelog included for details)

*Update*

Since I released HBL R97 yesterday, several owners of PSP 3000 and 2000 have experienced issues with the controls in their homebrews. The symptom: you run a homebrew and it keeps doing weird stuff as if you were pushing all keys like crazy.

Although I don't understand the root cause yet, this issue can be fixed simply by opening the file hbl/hbl_config.txt with your favorite text editor (for example notepad), and removing this line:

force_exit_buttons=0e00000009

Then save the file, and you're good to go. Of course, the "force exit" functionality will not work anymore, but I guess it's better than have your homebrews do crazy stuff. As far as I know, this issue only happens for psp2000/3000 owners with firmware 6.20. The PSP Go, in particular, is unaffected by this bug. We'll be working for a long term solution to this issue, but in the meantime, I wanted to let you know that we are aware of the problem.

By the way, you can follow the status of known issues here. This specific issue is tracked here