This by far is the number one reason to unlock your phone, apple deems fit to only us to background the phone, mail, and the ipod, but the iphone can definitely handle more than that! Especially if you have the 3GS but even for those with the 3G backgrounding is a super useful tool. It allows you to open programs and then switch to other programs leaving the original one open (in the background). Backgrounding is great when you set programs like Nimbuzz (to receive chat messages) and (Siphon to receive calls) to run backgrounded. Backgrounder is also nice for simpler things. Programs like the App Store and Cydia (the Pirate, “jailbreak app store”) or even certain games which take a long time to load. If you load them, and leave them backgrounded, you can flip back and forth quickly and easily.
2. Voipover3G
What can I say, its the greatest… this may not mean much for those on AT&T back stateside but Japan’s main carrier for the Iphone is Softbank, and they RAPE you on cell phone bills. Note the caps. With the basic plan, it costs 40 Yen a minute (46 cents a minute at today’s exchange rate). So for me, this is actually my number one because, nothing like being able to use VOIP over my data plan and not get raped by Softbank… maybe I will get my bill under 10,000 yen this month lol… doubtful 
3. Winterboard
Winterboard is just cool, you want 5 icons at the bottom? Yea they got that.
6? No problem, you’ve come to the right place!
Change your font, colors, icons, standard background, lock screen and almost anything else you can think of, yep, they can do that!
Its definitely the home depot for the iphone, lets you personalize it, definitely pleasing to the eye.
In combination with the Voip over 3G and siphone (or nimbuzz, though I havent been able to get Nimbuzz to work over 3G yet) GV is great, it basically allows my Japanese cell to recieve calls from my US GV number (routed through Gizmo to Siphon which is backgrounded of course)
5. Unlock
Unlocking my SIM card was very useful over Thanksgiving and I will use it again for Christmas, Unlocking your SIM is essential for any traveller, international rates are another way Softbank will rape you, international roaming fees! On the other hand, I don’t even think AT&T phones will be able to get service in Japan! Regardless of the country, with an unlocked iPhone, you can slide into any store and pick up a SIM card and be talking in no time, at local rates. It would be nice if you could get a data plan too, but they are hard to find on a prepaid SIM card. (Apparently Tmobile offers cheap prepaid data plans for 40 dollars a month in the US. They are over the slower Edge, not 3G networks though.)
6. SB Settings
This also could be higher on this list because its one of the most useful apps on my phone! Think windows task manager for your iPhone. Check running processes (damn backgrounded apps!) adjust brightness, toggle 3G, WIFI, Bluetooth, quick launch programs, see your stats, you name it, you can do it from SB Settings (which reminds me I have to put a link to siphon!)
Man, does anyone else hate Apple as much as I do? They are the new evil empire… M$ is cleaning up there act, securing up windows, playing nicely with open source and generally being less evil. Apple is building stronger walls and really playing games with their App Store. Why can’t google voice have an app? Who do you think you are? I am glad the FCC is stepping in and sticking it to them. Just wait, everyone loves them now, its hip and trendy to have a Mac… we sell out for the beautiful designs and stability, but that image won’t last.
8. Free programs galore
There are tons of free programs in Cydia, everything mentioned in this posting is free. Better still, if you get really deep into it and install SSH, you can steal any program available and install it yourself. That will be posted later, probably by Travis because I have yet to accomplish this myself
9. Siphon (SIP phone)
What can I say, free, awesome and it works over 3G, what else could you want? You can program any SIP service you have, so far I have only tested it with Gizmo (Google Voice) and Voipbuster, but its been great. Only feature its really lacking is a way to store your information for multiple accounts so you dont have to retype your info each time.
Siphon also backgrounds nicely, meaning you can sign in and leave it on to receive calls from people. I would love if they could find some way to hook into the regular phone notification system. Currently, when someone calls on my Google Voice number, it rings and I have to find the icon, reopen the app, and THEN click answer… total pain, and kinda slow. Anyone with info on this, please contact me!
10. SSH
More on this later, Travis will tell you more about it. Basically you can directly connect to your phone through your network and transfer files, do whatever you need. Straight up command prompt style. 
Thats all for now, any questions, or over programs you think I forgot, please feel free to comment below!