One useful thing my 5310 can be is an English Spanish dictionary. This saves me the bother of carrying around a paper dictionary or always having to play the "say a lot of words when it would be better to say just word" game. I have a couple of dictionaries I have downloaded for free online.
One dictionary is the Mobile Translator Me dictionary (the link shown when starting this program, eamobile.tk, is now owned by a squatter).
This dictionary has a nice interface; after hitting "5" twice to get past the two splash screens, it's a simple matter of choosing "Spanish->English" or "English->Spanish" then typing in, using the phone's predictive text features (it's identical to writing a text), the word I want to be translated. When talking to a friend who didn't know the word "Stewardess" in English at a cafe earlier this week, this dictionary correctly translated "stewardess" as "azafata".
However, its dictionary is limited. When translating "to melt" (as in, melting butter for popcorn), it translated it as "fusión", which really means "fusion" in Mexican Spanish. I had to use the KODi dictionary to correctly translate this word.
Another English Spanish dictionary I have is the KODi English Spanish dictionary. This program is pretty small, has an extensive dictionary, and opens quickly. Unlike any other dictionaries I have in my cell phone, it lets me words next to the word I'm looking up in the dictionary.
Unfortunately, the interface is a bit quirky; I can't use predictive text input to input words in to this dictionary, and, more annoying, if I hit the button which means "delete the letter I just mistyped" in this dictionary, it immediately exists the application, forcing me to restart it.
The third dictionary I have is one the official specs for my Nokia tell me shouldn't work in my phone: A huge 3-megabyte dictionary, complete with a lot of regional slang. The application works fine, even if it takes two or three seconds longer to load than the other compact dictionaries.
The slang isn't accurate slang for the Mexico City/Puebla area (the two naughty words they use for a female's reproductive organ aren't in the dictionary), but, then again, my ex-girlfriend who was from a small town about 200 miles from here didn't have those words in her vocabulary either, so I don't expect a dictionary to be up-to-date with those kinds of words. For words I would actually want to use in a normal conversation, it looks to be a good dictionary. It's biggest annoyance is that it's one-way; it translates from English to Spanish (which is what I normally need to do when talking to someone), but not from Spanish to English.
One annoyance all three dictionaries have is that they don't tell me the gender of a given noun; for a word like "té" where the gender isn't clear, I just have to give the word a male gender and hope it's a "boy-word" in Spanish. A verb conjugation tool would be nice too, come to think of it.
So, between these three dictionaries now in my cell phone (all three are free downloads), I don't need to lug around a big dictionary any more.