Dev Hook - Firmware emulator (lets you run version 2.0-2.71 firmware on your 1.5 PSP) Its also an ISO loader, and alows you to play UMD's that would require higher version firmwares.
IR Shell - A shell replacement app for the PSP, also acts as a remote for IR devices. Its an all round app, a bit like the unleashX dash on the xbox.
XFlash - Allows you to edit parts of the PSP that are based in the firmware, such as the boot up screen, sound effects and fonts.
PIMPStreamer - Two apps, you run the server on your PC and it converts you files in real time and serves them to the app running on your PSP over WIFI so you can watch all your movies from your PC on your PSP without a huge mem stick.
Custom Firmware POC - Its a custom firmware for the PSP that enables the PSP to boot up directly into devhook emulating 2.71. Theres a whole lot more to it too, theres lots of mods and plugins that you may find useful.
FileAssistant - The name says it all, allows you to manage your Mem Stick without the PC.
Downgraders - From MPH's first, to the recent Dark_Alex 2.71 downgrader, extreamly useful.
CSO-ISO-DAX Compressor - When loading your ISO's via devhook, it allows you to load them in CSO format, which is basicly just a compressed ISO image, this tool turns ISO > CSO.
PSP Set - Use it to Change your 1.5 wallpapers
PMP Mod - PMP is a video format (container) used on the PSP, this app plays them, very well and full res. Theres also a AVC version.
Theres so many more woth looking into such as emulators, but the above is what i tend to use.
Hope it helps.