I'd say that SnapTap (take pics in camera app with volume button press) and Activator (volume button, short hold) in combination have been an awesome combination. Why should WP7 and iOS 5 users be the only ones to take pics with hardware buttons? Lol
LockInfo has been the best so far for lock screen plugin implementation as well as home screen widget implementation. They also have a pull-down menu like the notification center feature that iOS 5 will have, but I won't use either most likely because I like using my status bar pull down gesture to be used for opening SBSettings. If you buy LockInfo, you get a free license for pop-up blocker, which is awesome for getting rid of notifications that you never want to have show up in the first place. The only ones I want interrupting me are calendar reminders and SMS alerts. And I use QuickReply for SMS, which is really nice, even though the developer has given up on supporting it and put it out as open source. Nice of him rather than just letting it die altogether though, I guess lol.
Parallax is really cool if you like the scrolling home screen wallpaper feature of Android. And I like using Iconoclasm for my Springboard layout. The style I downloaded for Iconoclasm is 'Boxed In Sparse' which spreads out my icons so I can see the wallpaper better :-) Even in iOS 4.3.3, though, there are problems with Iconoclasm and the folder system. It has a glitch where open folders will only be properly visible when they are arranged on the top row. Dev said it has something to do with folders having more apps in them than the number of row they are placed on, which makes them open mostly off screen. Weird, but not unbearable if you like the layouts and don't mind your folders being at the top of the screen only.
I use Call InfoFields to add notes to my contacts, which show up when someone is calling. Helps me remember who people are when I add someone with just a first name or just an email lol.
Clock Hide helps with LockInfo weather plugin.
IconRenamer is pretty self explanatory :-P you just make your icons wiggle, then tap and you'll get to rename them.
More Icon Folder allows you to choose how many columns of apps to display in an open folder, since it makes no sense to show three columns in a closed folder and four columns in an open folder. It also gives you the ability to go above the 12 app limit, though there are some glitches if you add too many. I lost my Settings app icon from the Springboard this way lol. It's okay though, if this happens to you: just add the Settings app to SBSettings dock or access it through the app switcher/multitasking view.
PkgBackup has helped me to restore my jailbroken apps and settings several times while attempting and completing iOS upgrades/restores. It saves a back up of your stuff in your contacts app, where you can either sync to iTunes or just sync contacts with Google through Exchange, the way I do ;-)
I also use Activator for some other cool gestures. I don't like double tapping the home button to bring up the app switcher/multitasking interface, so I assigned that function to all three of the swipe-up from screen bottom gestures. It is a gesture that works about as often as the swipe-down for SBSettings that I use; not every time, but you get used to how it works and it gets easier. I think you just have to make sure and swipe in as straight a line as possible lol.
Activator is awesome, so look into some of the plugins that are available for it. There are some plugins that allow using a gesture or button press to switch back to your most recently used app, like a back button only better ;-)