Free? I don't have my iPhone handy right now (it's a WP7 day heh) but off the top of my head my three all time favorite freebie tweaks are:
1) Weathericon - turns the weather app icon into a background updating picture reflecting the current weather condition and temp. First thing I redownloaded after jailbreaking this week.
2) Cascade (formerly Overflow) - turns the bottom dock into a mimic of coverflow in the iPod. Works best with Infinidock.
3) BlurriedNC - turns the notification centre background into a transparent layer with a blurred effect. You can customize the amount of blur and the colour tint of the effect. Makes it look like frosted glass is overlaid over your homescreen when you pull down the NC. Works on the lockscreen as well. LOVE IT.
FakeCarrier - Lets you change your carrier information (Verizon, Sprint, AT&T) to whatever it is you want it to say. Mine currently says iGinger because I'm a redhead and several people call me Ginger.
LSBroken Display - a really pretty lock screen display that shows current weather and time.
Arkansas Razorbacks 3G toggle - displays a Razorback where the 3G symbol should be.
iStar Signal Bars: Changes the singal bars to colored stars.
SJSharks Slider - Rather than Slide to Unlock it says Go Sharks!
Sharks Battery - Shows a Shark when the phone is plugged in.
1) LTE Logo for iOS 5 - replaced the 3G logo with LTE
2) Animated battery - adds a cool charging animation to the battery on the lock screen.
3) IconBounce - animates the dock icons
4) No Bookmark - stops safari from automatically opening bookmarks.
5) Suave HD - pretty popular theme and my favorite.
6) 3G Unrestricted - Enables apps that prevent use while on 3G. Also enables high quality video streaming over 3G for apps like YouTube and Netflix.
7) SBSettings iOS Notification toggles - Part of the SBSETTINGS tweak and it's great. Icons are big, you can move around the icons in which ever order you wish. I like this better than NCsettings mainly because the icons are too small.
IntelliscreenX (Amazing Lockscreen Application)
Remove Background (Closes all Multitasking Apps with a press of a button. No need to manually close each app in the dock)
Gridlock (Place your Icons where you want)
Infinifolders ( No limit for apps in a folders)
Winterboard (For changing themes)
Five Icon Dock (Speaks for itself
1 LS typophone4
2 close enhancer (very easy way to close an app)
3 swipeshiftcaret ( move the caret with slide gesture)
4 theme c'up (very nice free theme )
5 nc setting (its like a lite version of sbsetting that look really nice and clean in notification center )
This is my current lock screen, looks pretty sweet eh? Wallpaper is obviously mine but this theme is called TYPOPHONE 4. You'll find it easily by searching in Cydia. Enjoy
I just installed Update Hider.. hides any update that shows up in the AppStore... so if you dont want to update an app, you can hide the stupid update badge.