This post will mostly cater to owners of this phone, as I want to share a few good resources I've found.
A number of shortcuts, most of which seem to work. The coolest one is holding down the left soft key, which will read out loud any new text messages. You can go into tools / speech and download different voices and languages.
Themes for E71. Babi is apparently fairly famous as a theme maker, for good reason. The themes are free, and I really like some of them. Many are offered with a new set of icons, which I tend to think aren't worth it, and are rather large in size; versions using default icons are also available. There are countless sites offering countless themes, but making one that works well in all the different menus is very hard.
Nokia Beta Labs. A bit like Google -- try out new software functionality for free. Several of the free downloads are just as stable as the rest of the software offerings (which isn't saying much) and really well done. Sports Tracker is a common favorite.
Google tools, including Google Maps and Google Mail.
Free Nokia Downloads.
You may also want to go through pre-installed applications such as QuickOffice -- many offer a "check for updates" option and several have significant updates available for free right from the phone.