Must Have Software
There's an absolute ton of software available for s60 devices. I've played with a lot of trials, and wound up purchasing some, so here's the benefit of my experience.
Best Profiles: This does away with constantly having to change profiles on your cellphone, reacting on any number of quantifiers. For example, it can changeitub profiles based on which tower it's connecting to. When I'm at home, it's on the General profile, but as soon as I'm in the same area as my gym, it switches to silent. It can even scan your calendar for keywords, enabling the profile to switch on words like "meeting", or any other word you choose. As someone who's missed calls because he forgets to switch back to the General profile when he gets home, I find this very handy.
Handy Clock: I find this clock much more functional than the one that comes with the phone, mainly because it adds a timer and stopwatch feature to it.
Handy Calendar: Goes far beyond the built-in calendar, allowing for more settings and functions than the regular calendar.
Calcium: A fast calculator with a better interface than the one that comes with Nokia phones.
Panoman v3: Most phones come with version 2 of the Panoman panoramic photo software, but version 3 has MUCH better stitching. It's up to you, depending on if you take advantage of panoramic opportunities.
Mobitubia: Out of the other Youtube clients available, I find this one has the best interface and performance. I'm just hoping one with full access to my account and the ability to add favourites comes out soon.
Shozu: Imagine being able to take a photo or video with your device, and being able to upload it via Wifi or your phone's data service to any of your media sharing sites. Youtube, Flickr and Facebook are available, just to name a few.
Coreplayer: One of the better video players on the platform.
Y-Browser: A better file manager than the one that comes with the device.
Fring: Even though it's not perfect, it's probably the best multi-protocol chat client (in my experience at least) for S60. The official MSN Messenger client has a great interface, and great options, but I find it drops a lot of messages.
There's some other handy software depending on what you want to do with your device. Handy Safe is great for keeping track of personal info, passwords, logins, and so on. Moby Explorer has some basic FTP client functionalities. Alon makes a good dictaphone / call recorder. There's plenty of great software, and many companies offer free trials, really showcasing the kind of developer community Symbian has.