Right now I have two X-terminals and xchat open. The x-terms because I like to monitor CPU and memory use so that I don't overextend the device's limited resources, and to monitor the battery because the original meter isn't that helpful in helping my ensure I don't run out.
Xchat is of course for keeping in touch with what's happening on the intarwebs.

This is my "minimal" setup. When I'm asleep, or in my freetime when I'm not using the N900. Sometimes I go to sleep with media player streaming something relaxing from internet radio, with the "Sleeper" widget from maemo.org extras repository.

At my dayjob I'll typically have the above plus gpodder and media player. I listen to various news and music podcasts during work, sometimes browsing internet too in the quieter patches of work.

At my currently second semi-volunteer job I actually need to communicate, so I'll have Conversations open to do directory lookups via sms, phone app open, Notes app open to take quick notes notes because those pens just vanish in my car, Ovi Maps and Sygic Maps open. It's easier to find stuff on the map with Ovi Maps, but it's harder to navigate there with Ovi Maps than with sygic maps. When I've found something with Ovi Maps, I usually try to find it again with Sygic, but Sygic has different idea of what every place is called, so it's not always easy. I like Ovi Maps feature of basically having free-text search of the globe, whereas sygic requires a city name (and it has a weird idea of that, not even locals are familiar with the naming sygic uses), or post code, which isn't usually mentioned in the addresses I get.

Especially the latter scenario stretches the phone's hardware resources to the limit with two GPS apps open, plus the phone and sms apps, and the apps from my "minimal" set. Sometimes I also need the browser on top of that, which usually means a half-minute pause for the page faulting and swapping in and out of stuff...