Here are two download links for the new 3.51 M33 Custom Firmware:

For users installing from a custom firmware: http://www.sendspace.com/file/a5plbr


For users installing from 1.5: http://www.sendspace.com/file/01ievx


Also another easy installer (psEvolve): http://www.mediafire.com/?aow4jpzgl1e
Mirror1: http://www.sendspace.com/file/qiqfwa
Mirror2: http://www.megaupload.com/?d=Q3GHS4D6


Summary:
All functionalities of official firmware 3.51 (Support of plays PS1, video high resolution, visualisor MP3, navigator Internet, connection PS3 starting from Internet, etc…)
Launching of UMDs official asking a going firmware until the 3.51
Direct launching of ISOs/CSOs since the small PLAY > Memory Stick
Direct launching of homebrews and demonstrations since the small PLAY > Memory Stick
Direct launching of plays PS1 (converted with format PSP) since the small PLAY > Memory Stick
Possibility of regulating the speed of the processor of the PSP
Recovery Mode
Multizonage
Many bugfixs
Possibility of customiser all your icons/topics/vague etc
Install:
1.Backup memory stick
2.Format memory stick
3.Add the PSP folder in the .rar to the root of your memory stick.
4.Launch the eboot in your xmb
5.Follow directions and reboot when prompted