You can find SIStore here, with a direct link to a self-signed installer for an on-device client, giving on-the-go access to obscure apps and games. Here's SIStore in action on my 808:
There's no checking for what's already on the phone, mind you, this is simply a SIS archive browser. So it's up to you to know what you have and haven't already got installed! And there's also no update mechanism, spotting new versions, of course. So all a little primitive, but at this stage in Symbian's life (i.e. it's been obsolete for almost half a decade) any activity and any archive source is helpful. Especially as there seems some impetus here from active Symbian users to find workarounds for things which have stopped working.
It's not clear how this will behave on phones with production firmware (my 808 has Delight CFW), so comments welcome, let others know how you get on!