What to make of my unused GT-I5800
a media server, using e.g. BubbleUPnP
a machine for development/coding, using e.g. Terminal IDE
a "picture frame" (before mentioned BubbleUPnP should be able to do that along, but many gallery apps in slideshow-mode as well)
a web server, e.g. with AndroPHP / Bit Web Server (PHP,MySQL,PMA) / Ulti Server: PHP, MySQL, PMA – also see: Turn a Android Phone with broken screen into dedicated Web server
have it ready as emergency/backup phome with some prepaid card
use it as "modem"/router for devices without a SIM card (tablets etc.) or without a data-plan, utilizing its hotspot functionality (sure you could use your main device for it; but using the old one keeps the juice in your main device ;)
use it as "test device" for any apps you want to try. With no important/daily-use/private data on it, you could even use some "risky sounding" apps here, and easily factory-reset it afterwards. For this, a separate Google Account might prove useful as well.