These items I pretty much use all the time.  THree are always with me and the analyzer I usually just bring if I'm running sound.

Peterson Strobe Tuner

Changing strings or matching keyboards and guitars etc are no longer a chore.  A 12-string is just as easy to tune as a 6-string. Obivously takes a few moments longer, but it's fast.  Wire it, use the mic, whatever.  Best piece of kit I own.

Next up, Behringer Cable Tester

Not only checks cables but identified exactly what the issue is but can also id intermittent connections.  You'd be surprised at how many XLR cables are not wired correctly.  They will still work, but there can be noise and phase issues.  Also has a pretty flexible tone generator great for chasing down signals.

Finally my trusty Behringer Signal Analyzer

Obviuosly only needed when running sound, this unit is generally marketed as an EQ, but it's real claim to fame (and the reason you see them everywhere in sound live sound installations) is the Real Time Analyzer.  It's got a lot of bells and whistles, but the best part of it is plugging a mic into the back of it and hitting the RTA button.  I have many times thrwarted problems before anyone heard them.  In the studio it's a great in line reference for the overall mix.  A 2nd set of ears while mixing and mastering if you will.  Doesn't change anything, but lets you "see" the over mix.  There are several other tools that do all of this, but this one is fairly inexpencive and does it's thing really well.

 Finally, this Behringer DI

Another item like the Strobe and Cable Tester that is ALWAYS in my kit. up to 40db of attenuation, ground lift, Phantom Power and supports input from Tip Ring, Tip Ring Sleeve, and XLR.  Can even use it as a splitter to a personal monitor.. All in a solid cast aluminum indestructible box.  I carry two with me all the time and can hardly remember a gig that I didn't need it for something. 

As far as items that I don't use...  I really can't think of any.  I usually research things fairly completely, get references, consult with collegues and other professionals who actually use the items.  Only then do I actually make a purchase.

Best:  Empress Compressor

Amazing Compressor with sidechaining capabilities this one does an exceptional job of evening out your tone and level without degrading the sound (although you can make it quack and smush the sound).  Works great for either recording or live giging.

SOURCE AUDIO Programmable EQ

This is a great accessory that allows you to fine tune the frequencies to best suit the venue.  I have 4 different settings programmed in and one of them will usually doing the job.  Great for removing the annoying overtones and works incredible sidechained into the Empress Compressor.

ZOOM Q3 HD recorder.....simple to use, excellent sound quality and amazing video all in a small package with good accessories.

BLUE CHIP PICK....closest thing I have found to real tortise and virtually indestructable

WORST:  MusicPadPro......dated technology that was overpriced forever but the only available option at the time.  Replaced by easier to use multipurpose iPad2.