This is one of the more famous shells for PSP.

Changes:

Supports the latest M33 firmwares across both Phat & Slim. Slim users can choose among 3.60M33, 3.71M33-1 to 3.71M33-3.
PSOne 2-Player Adhoc Support.
Introduces a new Context Menu System. Functions (plugins) under context menu will always be available to all file types, similar to the right mouse menu click under Windows.
Added a new Multi-tasking PRX Plugin System. This allows user written PRX Plugins for iR Shell to multi-task with another game/app. The included PRX HTML/Text Viewer developed by korDen allows you to view HTML & Text documents while playing games or homebrews..
With the help from Dark_AleX, support to M33 & NP9660 No UMD Modes have been added. NetHostFS & UsbHostFS are also available for the No UMD modes.
Added support to the latest Popsloader for 3.71M33-3 by Dark_AleX.
Allow random start of MP3 for the alarm clock function, so you’ll be greeted with different tunes each morning.
Added the options to put your PSP to auto-sleep while idle to save battery.
Allows you to press Hot-Key to turn off/on all LEDs. The annonying memory stick or WiFi LEDs will no longer distract you from watching your favorite video/movie.
Support TV-Out for Slim via component cable.
Added the display of volume bar for PSP main volume in iR Shell.
plus numerous bug fixes.

http://forums.qj.net/f-psp-development-forum-11/t-release-ir-shell-38-128231.html