Here are some of the applications I use:
HandyShopper - this is a database program, originally designed to do shopping lists, which is what I mostly use it for. It let's me organize things by store (and aisle if I want) and can keep track of the prices things cost. I also have a separate database for gift ideas, so when I'm struck with a gift idea, I can note it down and remember it later.
Countable - I knit and crochet. Lately since I have so little time, I do a lot of it when I'm in the car travelling (hubby's driving). Countable is a counter that let's me keep track of rows, increases, decreases, pattern repeats etc. Mostly I just use it as a row counter. With kids, I get interrupted a lot while I'm knitting.
Bible+ - The Bible on my cell phone, so I can have it with me all the time. I can download different translations, jump to book and verse, and search for keywords. Even see different translations in parallel.
Memorix - I stumbled on this one this past Christmas. It's a memory game where you "turn over" the cards and make matches. Great to entertain the kids when we're waiting in the store, at a restaurant, etc.
IM+ for Skype - I'm not sure if you're familiar with Skype, but it's a service that allows you to make phone calls from your PC. If you're calling to someone else who has Skype, it's free. There's also a text based chat/IM method of communicating. (I think I might talk about Skype next week, tune in!). We use it a lot at work, since our offices are spread out, we need to do a lot of conference calls and we use the chats a lot when we need to ask quick questions of each other. IM+ for Skype isn't made by the people at Skype, but they don't have a PalmOS client, so I had to turn elsewhere. It's nice because I can answer questions from my employees while I'm away from my desk (ex when I'm at the pediatrician's office with my kids). IM+ for Skype is one of the few programs I've actually purchased for my phone.
Pocket Quicken - Pocket Quicken is the other program I've paid for. It's a version of Quicken (accounting/financial software) for Palm OS. Hubby likes to keep track of all our expenses, receipts etc. I was TERRIBLE with loosing receipts and not getting them to him. I'm still bad when I'm in a hurry, but with Pocket Quicken, I can enter them into my phone right away or while I've got a spare minute and then Hubby just sync's my phone with his system (which has the full version of Quicken on it) and imports the data. He's much happier now :) Unfortunately, it wasn't made by Intuit and Intuit has terminated their agreement with the company that was making it, so you can't get new versions that work with current Quicken software.
Unit ConPro - At work, I frequently need to convert between bytes, megabytes, gigabytes with byte accuracy (1 GB = 1073741824 Bytes) I could memorize the numbers, I could use the calculator in Windows, or I can use a unit converter like Unit ConPro (I also use the WWW Unit Converter, but lately ran into a bug with it). It also will do conversions for Volume (cups, pints, teaspoons, etc) which comes in handy when you're doubling recipes, or trying to figure out which item at the grocery store is bigger. It also does a whole pile of other unit conversions. If you can think of it, I bet it does it!
pssh - An ssh client for PalmOS. For us tech folks. I don't use it often, because most of the systems I need to ssh to aren't accessible via the internet and I don't want to install a VPN client on the phone (although they are available).
Documents to Go - I actually don't remember if I paid extra for this (I did for my old Sony Clie) or if it came with my phone, but this allows me to create/edit Office documents (Word, Excel, PowerPoint and plain text). It also allows me to view pdf files. I mostly use this for recipes. I've got recipes that I store as plain text files on my computer and when I'm planning on making one, I put it on my phone so I don't need to print it out to make it. You can also get some books from Project Gutenberg in plain text, although I find them a little hard to read that way.
Google Maps - We don't have a GPS, but I can get easy access to Google Maps from my phone and it can approximate where I am (based on which cell tower I'm using), sometimes with better accuracy than others. But it's nice having access to the maps that Google provides when I'm out and about.
The other software I use is stuff that came with it, mainly the email client (which does pop and IMAP, although I've been having issues with it lately), the web browser (nothing special really), and the MP3 player (which I talked about on my audio book post).
Overall, my Centro is really working for me. I'm hoping that there will be another good PalmOS based phone out when I'm ready to replace this one. Check out other Works for Me Wednesday posts at Rocks in My Dryer.
