Notable stuff I've cottoned onto since upgrading to 6.1 (and prior, to an extent):
Bars (Free, improves signal strength meter by having increments of bars rather than just the five bars)
Bridge ($2, import media directly into your iTunes library on your phone, good if you download MP3s in a third party browser)
iCleaner (Free, gets rid of unnecessary crap that's never used, like iPad/non-retina images etc., freed up at least 500mb just this morning)
iFile (Not free... only a few $ I think, file system browser/editor w/ web server and now hex editor)
Infinifolders (Not free, cheap though, have unlimited icons in folders)
iSHSHit (Free, back up your SHSH blobs in case of... something, like phone recovery I think)
Protube/HD (~$2 I think, Youtube player/downloader that's well supported and rock solid. HD is for iPad, obv)
Springtomize ($3, is basically a collection of tweaks in a single app, things like icon (rows, labels, size, colour) and task bar customisation, amongst a shitload of other things. Great value and a must-have, IMO)
Zephyr (~$2 I think, adds super slick app transition and app switcher activation that complements Auxo really well)
3G Unrestrictor ($3 or so I think, lets you use wifi-only apps over 3G/LTE, can customise per-app which ones should and shouldn't bypass the restriction)
UpsideDown (Free, will let all apps run while your device is upside down. Kinda useful for me, personally)
I used to use Swipeselection, which allows you to move the cursor left/right when using the on-screen keyboard, but it got a little bit finicky and caused typos here and there when it moved because I typed too quickly.
KBShortcuts is supposed to be a worthy replacement, but I've not used it.
That's basically a list of what I have installed on my phone at the moment. There are a couple I'll probably add back on later (Infiniboard maybe, and a colour adjustment thing from Ryan Petrich) but to be honest, I'm happy with how it is. Of course there's heaps I *could* do (haven't even mentioned Winterboard) but any more tweaks would just overcomplicate things IMO.
Edit: Shit,
none of those are lockscreen tweaks. Springtomize has a few basic ones, but that's it. But hopefully there's some useful info in there for ya.