These are the best all-around jailbreak apps you will find on Cydia that serve purpose with great funtionalities. There are some great tweaks add to this list as well.
Activator: A really useful app that you can use to completely customize how to open applications. Volume buttons can open up the camera, gestures can open up your SMS. You could also do something crazy like holding down the power button to make a call to your Mom or Dad if you'd like. You can use it to launch anything on your phone. It is your choice what functionalities your iPhone has at this point.
Backgrounder: Apple implemented a quasi-multitasking in iOS 4 that's been good enough for most. But if you still want authentic, seamless and just good ol' fashioned multitasking, Backgrounder has always been the best option.
3. bigU Movies: (Allows you to watch over thousands of full length new movies and dvd movies in very high quality. A must have app if you want to catch up on all the recent movies you missed, want to watch new movies, you have tons of free time on your hands or if you just want to stream movie from this app to your TV for the whole family to enjoy.)
BiteSMS: It's a significantly more powerful messaging app and includes features like smileys, privacy, scheduled SMS, passcode lock, delivery reports, signatures, and more. The star feature is Qucik Replying, as you can reply without ever getting into an app.
iBlacklist: Easily block calls and texts from people you don't know or just don't want to talk to.
Lock Calendar: For those who like useful lock screens, Lock Calendar displays your calendar on, well, your lock screen. It's reminiscent of 'Today' home pages of Blackberrys and Windows Mobile devices.
MyWi: You can turn your iPhone into a 3G hotspot without paying AT&T or Verizon another dime. Sure, there's some grey area legally speaking but you won't complain when you're using MyWi instead of paying a hotel 20 bucks a night for crappy Wi-Fi.
My3G: My3G tricks your phone into thinking it's connected to Wi-Fi when it's actually on a 3G network. So you can Facetime over 3G, run higher quality videos in certain apps and do things that are Wi-Fi restricted. Crucial.
Notified Pro: Like Mobile Notifier and Notifier+, it's a better notification system for iOS. Though soon to be obsolete when iOS 5 releases (it's nearly the same solution), it delivers non-intrusive webOS style notifications and an Android style drag-down banner.
Pocketmode: It geniusly uses your iPhone's ambient light sensor to determine where your iPhone is. So if it's in your pocket (dark), the ringer is customized to be louder than when it's on your desk (bright). And don't worry, there's a night mode to prevent it from shrieking during your sleep.
SBSettings: This is a must have if you own a jailbroken iPhone. SBSettings gives you easy access to all the settings you'd like to adjust (Wi-Fi, Bluetooth, Orientation Lock, etc.) by just swiping down the Status Bar. No more digging in menus! It's totally customizable and accessible from anywhere. It makes no sense that Apple hasn't done something like this yet
Springtomize: A suite of custom tools and tweaks that let you adjust your iPhone like icons, folders, status bar, lockscreen, animations, springboard and more.
Winterboard: This app is a must have and a classic especially if you want to customize almost everything on any iPhone. We're talking about crazy themes, wacky fonts and anything else you'd want under the sun.
5-Row Qwerty Keyboard: It adds a row for numbers on the keyboard. Definitely a time saver.