SBSettings/Activator - these sort of go hand-in-hand, and are generally suggested to be the first things you get from Cydia, as many other tweaks depend on them. Among other features, with SBSettings I can now swipe my status bar and get control of a variety of settings, like brightness and switching wifi on/off. (My iPad and router don't always agree, so being able to reset wifi with a touch is a huge improvement for me.) Also note in the screenshot below that my status bar shows not only the time but the date and how much memory I currently have free - also due to SBSettings!
Activator, on the other hand, allows me to assign meanings to a variety of actions - for example, a certain swipe on the screen brings up the task switcher bar, without the possibly-baby-waking noise of a double-press of the home button.
NoLockScreen - What it says on the tin. A lock screen makes sense on a phone to avoid embarrassing butt-dials. But when was the last time you hit the home or sleep buttons but didn't actually mean to turn on your iPad? If you're tired of sliding to unlock, here is your fix.
BrowserChanger - If you use a non-Safari web browser like Atomic or iCab (and if you don't, you should check them out), you're no doubt frustrated that links in all other apps still open in Safari. Fixed!
RetinaPad - Now we're getting into paid packages. You know how Apple cripples iPhone apps on the iPad, not allowing them to run at full resolution even when you hit the 2x button? They did this for an okay reason - to encourage developers to create real iPad versions of apps instead of just resting on their iPhone version. But now that the iPad is a bona fide hit, it's kind of silly to keep the pixelated mess - RetinaPad will fix that for you.
Dimmer - I actually just bought this tonight, and I don't know what I was waiting for! If you've tried to use your iPad in bed (say, during 2am baby feedings, not that we know anyone doing that right at this very moment...), you know that the brightness controls really don't go down that far. Dimmer lets you dim the screen as far as you like - and I am loving the results so far!
iFile - Direct access to your iPad's filesystem. Don't use this one unless you consider yourself clueful enough not to move or delete anything important! This has been really helpful to me in getting some word and excel files back and forth to DropBox from apps that don't let you do it directly. The downside is that I've discovered just how many apps don't really store your work in individual documents that can be easily moved around!
I've installed a few other packages, but these are the ones that I actually use regularly and that have a real impact on my iPad experience. Do you have any to suggest?