PlayStation Portable

Custom Firmware
Every model and every firmware PSP can have a custom firmware installed to it. The process for all of them is to upgrade your PSP to 6.60, put the Pro CFW installer on your memory stick, and run it. It isn't a 'permanent' CFW by default, so if you power off your PSP then when you next turn it on, you'll need to run the Fast Recovery tool from XMB, which only takes a few seconds. It IS possible to make it a permanent install on 1000 and early 2000 models, but if you just keep your PSP in sleep mode rather than turning it off, it'll never be an issue anyway.

Tutorial and files here http://wololo.net/cfw4dummies/

MicroSD Memory Stick Adapters
Not exactly homebrew, but good to know; you can get Micro SD to memory stick adapters, they come with two slots and cost a few dollars at most. You get way more space for a fraction of the price. 32GB is possibly the limit per SD card, for a total maximum of 64GB.