If you want to install custom applications on your Nokia 5800 XM, you’ll have to sign them with a certificate…
or just hack the phone and sign nothing.

Before you can start, you have to get a developer certificate especially for your phone.
Visit cer.opda.cn , register, login, goto My Certificates and press Apply cer.
Enter your information, you’ll find the IMEI number by typing ‘*#06#’ into your phone.
Your .cer and .key files will appear on My Certificates after about 24h.

Download the HelloOX2 Unsigned package from helloOX2.com
Now you have to sign the .sis package with your new developer certificate.
There are several ways to sign: Online signing (f.e. cer.opda.cn), various Windows tools,
symbian sdk kit for linux which includes a signing tool, or directly on the phone (f.e. FreeSigner).
The FreeSigner tool is already signed with a certificate (.sisx), means you can install it without any modifications.

Before hacking, backup your personal data on the phone either over Windows and Nokia PC Suite,
or use the internal backup tool from your phone and then backup your memory card.

Transfer your (un)signed HelloOX2 package to your phone via Nokia PC Suite
or manually to MEMORYCARD/data/installs, unplug phone and then install it on the memory card.
Just follow the popups (Modo is a file manager, if you need it).
Remove HelloOX2 if you like (but not RomPatcher+ !).
Afterwards, start RomPatcher+, activate Installserver_FP2 and add it to Auto.

Your phone is now hacked and you can install unsigned applications.

All credits to HelloOX2 and RomPatcher+ team.

Update 1:
Sadly my phone has an old product code, the new 40.0.005 is not available for me.
But updating from 31.0.008 to 31.0.101 still works with HelloOX2 (hooray….)

Update 2:
After version 50.0.005 upgrade, apps and rompatcher+ are still working.
No reinstall required, but if you would like to, consult HelloOX2.com

Take a look at these pages for apps, themes, games, etc.
Symbian Underground
MobileCastle – S60 v5 Symbian OS 9.4 Applications
DailyMobile – Symbian S60v5 Forum
GizmoGates – Symbian S60 5th Edition Application / Games / Themes