 Update: new hacks for Nokia and SonyEricsson phones with S60v3, S60v5, and Symbian^3. No certificates needed!
Update: new hacks for Nokia and SonyEricsson phones with S60v3, S60v5, and Symbian^3. No certificates needed!
Most Symbian phones refuse to install "unsigned" software. Programs need to be "signed" with a digital certificate before Symbian allows you to install them on your very own Nokia or other Symbian phone. If computer programs would have to get approved by something called 
Windows Signed there'd be public outrage, and Microsoft would be flooded with lawsuits. But for some reason Symbian (read: Nokia) gets away with it.
Commercial Symbian software usually comes pre-signed and ready to install on your phone without further ado. But getting software signed for distribution requires a Publisher ID, which costs hundreds of dollars. For obvious reasons most authors of non-commercial software are not going to buy expensive Publisher IDs to sign their freeware.
A year ago 
Open Signed Online was started to cater for the creators and users of non-commercial freeware, but so far it fails miserably. Open Signed Online is still in beta testing and it shows. The procedure is unnecessarily complicated, especially if english is not your native language. It requires each and every user of unsigned freeware to upload the files to be signed, hand over their IMEI number and email address, wait until Symbian Signed emails back to them, respond to that mail, wait for another mail with a download link, and then return to the Open Signed Online site to download a signed copy of the program. Open Signed Online won't work on large files (the current limit is 4 MB), is not available in any language other than english, and its web site is unsuitable for mobile phone web browsers.
And if you want to install another program, or the same program or another phone, you'll have to suffer the same ordeal again.
As a result, Symbian Signed protects the financial interests of a few commercial software companies at the expense of all the non-commercial programmers and the millions of people that use their freeware. As long as things don't improve, make sure to avoid Symbian Signed like the plague.
So what are the alternatives to Open Signed Online?
You can 
set your phone free with one of the Symbian hack applications and use the 
SIS Installer Mod to avoid the inconveniences op Symbian Signed. Unfortunately you can't hack new phones with new firmware, so you can't use the SIS Installer Mod on those phones.
Fortunately there are free programs to sign .sis files without taking the long and winding road of Open Signed Online.
Programs to sign Symbian .sis installer files on your computer:
• 
SISContents
• 
FreeSigner
• 
SignSIS
Programs to sign Symbian .sis installer files on your phone:
• 
FreeSigner
• 
MobileSigner
Of course you'll need a certificate to sign .sis files with these programs. And if you want the signed applications to have enough capabilities to be useful, you need to sign them with a Developer Certificate. Symbian Signed won't give you a DevCert, so you'll need to get one elsewhere. You used to have to struggle with chinese websites to get certificates, but recently they've become available on english sites too.
Not all certificates are equal. The certificates from OPDA can give your programs DiskAdmin capability. Certificates from other places may not give your programs all the capabilities they need.
Places where you can get your own free certificate (these certificates give your programs "17 capabilities," including DiskAdmin, which you need for the 
HelloOX method):
• 
OPDA forum
• 
OPDA certificates (english language version)
• 
iMobile
• 
guide to getting certificates from iMobile
• 
DevCert Generator, an english language frontend of 
cer.s603rd.cn (sometimes this site doesn't work, sometimes it hands out certificates without DiskAdmin capability, but sometimes it gives certificates with all required capabilities to sign and install HelloOX)
• 
Nokioteca (italian site)
• 
Nokioteca guide on Symbian Freak (login/signup required)
• 
SIGNSIS
• 
S60 Certificate Key, another english language frontend of 
cer.s603rd.cn (sometimes this site doesn't work, sometimes it hands out certificates without DiskAdmin capability, but sometimes it gives certificates with all required capabilities to sign and install HelloOX)
If s60certkey.com asks you to download a toolbar, click "cancel." After the page reloads, right-click the cer and key file links and select "save as..."from the menu. This way you'll get the cer and key files without the toolbar.
Publisher IDs get revoked, certificate makers go on- and offline, and sites that work today may no longer work tomorrow.
Keep in mind that your developer certificates are made for you for free by friendly volunteers, so please behave accordingly. If you get your developer certificate from a user forum, don't just go there to grab a certificate but try to contribute something to the forum as well.
Update: new hacks for Nokia and SonyEricsson phones with S60v3, S60v5, and Symbian^3. No certificates needed!