Nokia just launched Nokia Forum Wiki.
Via AllAboutSymbian, we also have Sony Ericsson Developer Wiki and the Symbian Developer Wiki.
I did a 20-minutes google search and found out there are more out there:
Adobe Labs Flash Lite Wiki
Channel9 Windows Mobile Developers Wiki
HowardForums Wiki
And more from device manufacturers:
xda-developers wiki: HTC devices
Motorola Q Wiki
Wow. I am totally amazed by my findings 
77 Windows Mobile devices are supported!
Wait, most of them are labeled as “coming soon!”. My T-Mobile Dash is one of them .
Via Channel 10 - on10.net
It is still in prototype stage, but looks very promising.
I thought today I can download the smart2go client and give it a try. Apparently that site is overloaded and gives a MaxTransientObjectsExceeded error.
But I happened to find some smart2go slides on Flickr. Here are two of them:
Yet another free map/navigation application for mobile phones. It is from Nokia, but it supports both Symbian and Windows Mobile devices.
The platform allows for mapping and routing in over 150 countries and has support for full turn-by-turn satellite navigation in over 30 countries. The application allows people to view where they are on a map, search for points-of-interests (POI) around them and create routes to get them there free of charge. Nokia plans to start offering the smart2go application for free, on Saturday, February 10th, for selected Nokia S60 and Windows Mobile 5.0 devices initially and has plans to roll out support for most of the major mobile OS platforms including Nokia S60, Series 40, PocketPC, Linux and other Windows Mobile devices.
Open Source Software for Mobile Phones