A few more to add to the list..
For these ones you need your device to be rooted. Some of them are the free (limited) versions, but still give you a wide range of options. I was able to back up and restore data from my phone to my tablet using Titanium free version, so it's all good. Comes with a stack of extras in the paid version though.
AnTuTu CPU Master (overclock your CPU)
Droidwall (just allows you to control the data flow of your apps if you're on a limited tariff)
Root Browser (as it says..)
Titanium Backup (backs up everything on your device - and does a bit more.)
Smart Ram Booster (kills unneeded background apps.)
SD Maid (all in one cleaner for android, etc.)
Greenify (helps control unruly apps)
Battery Calibration (resets your battery stats - this worked wonders on my phone)
Some general stuff.
AirPush Detector (detects the apps which push the annoying pop up ads)
Electrum Drum (make beats!)
ReLoop Loop Sequencer (made by the author of Electrum Drum - does what is says on the tin)
Sonic Chop Sample Ripper (same person again, and does what it says)
(The above three demo's are quite limited, but cost around £6 for the three of them..)
Gnaural for Android (you can't edit your own binaurals with this, but just copy and paste you folder from your PC. I'm having issues playing .wav files, but just waiting for the author to get back in touch)
Swiss Army Knife (unit converter, spirit level, timer, etc.)
Aqua Mail (for hotmail, yahoo, etc. etc.. nice clean interface, and very quick)
PDF Viewer (best one I've found so far)
Gstrings (guitar tuner)
Adblock (google removed it from the market, so you'll need to go
here to get the .apk
Twilight (adjusts your screens colour temperature over the course of the day.. or night)
TV Catchup (as it says, but you need to be in the U.K. to use it)
My Boy! (GBA emulator)
Roms
Bubble Defence (decent tower defence game)
Stickman Downhill (addictive and fun bike game)
I've got these on my devices, so I've tested them and they work pretty good for me..
And that's about it for now