I recommend Navit, but you have to dig around in the Navit thread to find links to the latest version (the one in the repos is a bit old).
Cloud GPS is actually coming along nicely, but still no voice navigation. (However it can route internally, instead of having to connect to the internet to find a route.)
Ovi Maps (the preinstalled mapping software) CAN be hacked to be a lot better. There's some development on making it have voice navigation in progress - not the program itself (as it's closed source so we can't modify it) but by making another daemon run in the background that detects when the maps program is about to come to a turn on the route, and plays a sound file.
Then there's Mappero/Maemo Mapper, and a couple of other not-bad programs in development - but they all (some have voice navigation, some don't) can't route internally, they have to connect to the internet to do so.
not free, but sygic mobile maps is easily the best I've found. Theres a 14 day trial in the OVI store. Actually works like a proper GPS with turn by turn navigation
I would recommend modrana. Internal routing is not there yet, but I believe it is being worked on. There is voice navigation, and support for offline map storage. And unlike sygic it's free.
Well since Nokia abandoned Maemo we will never see the Ovi Maps updated.. Sadly.. Unless they actually do get the Qt version of Ovi Maps out, then someone does port it back to Maemo..
There's turn by turn voice navigation in Navit. Which is why I recommended it.
if modRana is working on internal routing, great.
Right now the best option in my eyes that has "everything" (turn by turn, voice navigation, AND internal routing) is Navit - the only other one that has the same features is Sygic, which isn't free. modRana, CloudGPS, and Maemo Mapper/Mappero and I think one or two others, are the next in line - they all have either turn-by-turn navigation with voice, or internal routing. Most don't have both, annoyingly.
Oh, and don't call voice navigation "turn-by-turn". Ovi Maps technically has turn-by-turn, as does just about every mapping program on Maemo - it shows you when each turn is - it's the voice part, where it announces those turns out loud, that's missing.