#1 Firefox - Easily my favorite way to surf the web, on the N900 or otherwise. If you want more of my thoughts on this one check out my Firefox Mobile Review.
#2 fMMS - This application steps up to fill one of the biggest mess-ups Nokia made when they released the N900: lack of MMS support. fMMS currently supports sending pictures and receiving all types of media messages. Since I discovered this wonderful application I no longer have to hear my friends say "your phone does all that, but can't get a picture message?"
#3 TweeGo - Easily one of the most professional looking applications in the repositories that was created just for the N900. The UI is pleasing to the eye, easy to navigate, and provides everything you need to fully use twitter.
#4 FaceBrick - Currently still in development, FaceBrick just recently released its 0.3 version. It supports viewing your news feed, posting/viewing comments on posts, and posting your own status updates. This is for sure one to keep your eye on and I have no doubt that within a couple of months it will be just as polished and feature rich as TweeGo.
#5 Conky - Ever wonder what application is eating up all your system resources? Or maybe want to quickly check exactly how much rootfs space your device has left without having to crack open a terminal? Conky is your one stop application for all system monitoring related tasks.
And finally, because I want to include a good N900 game on this list, I'd like to include my girlfriend's pick for best N900 application: Angry Birds. Seriously, if you own an N900 and haven't downloaded and tried this one yet, stop reading this and go get it now. At least an hour of classic video game fun in that one there :)
What are your favorite N900/Maemo applications that you use everyday on your device? Looking for some other useful applications for the N900?