SignMe! is a small piece of software designed to make the whole sis files signing process a bit easier. Simply right-click on any .sis file, chose "SignMe!" and that's it.
(To make things clear - you'll still need to sign up at SymbianSigned in order to have both .cer and .key files. You'll be prompt for them and the certificate password the first time you'll run the application.)
Free Download: signme_beta_107.msi