Label Maker—Every studio needs a quality label maker. Having easy access to one comes in handy when labeling hard drives, gear recall settings, or microphone cases. I use the Brother PT-1090, but just about any label maker will work. You can pick up a great label maker at any Wal-Mart, Office Max or Lowe's.
Gaffer's Tape—Every live engineer on the planet has rolls of this stuff. Gaffer's tape is like "Duck Tape" but it doesn't leave a gooey residue. Use it to tape down cables so things look neat and tidy, and also so no one trips. Gaffer's tape is like a drug for me. Sometimes I gaff things that don't even need to be gaffed, just because it makes everything look so nice.
Cable Ties—You'll want a large stock of plastic and Velcro cable ties. I use black plastic ties to help organize cables permanently behind my desk. The Velcro ones are used for microphone and instrument cables.
Solder Gun—Cables go bad and connections come lose. Having a quality solder gun around can really be a life saver. If you've little experience with a solder gun, you can find tutorials on the internet and on YouTube that show you how to solder cables. I use a Craftsman Professional dual-heat soldering gun. There are even cold heat guns too that won't burn you if you touch their business end. Maybe a good idea for the clumsy musicians out there! I've had my fair share of solder burns. (Ouch!! Not fun!)
Extra Rack Screws—Rack screws get lost just like guitar picks. You can buy a whole box of high quality Raxxess rack screws for about $13.00 for 100 screws. This way you'll never be without.
Cordless Drill—I find it incredibly useful to keep a cordless drill in the studio. From removing rack screws quickly to removing wall plates. You'll save time by keeping a drill close at hand. I have a Craftsman 17191 Cordless Drill. One of the features I really love is the ability to lower the drill's power, so that I don't strip rack screws. Keep an assortment of drillbit ends. You never know what you'll need. An assortment of Phillips, straight-head and hex-heads is a plus. Here's a tip, buy a drum key drill bit for speedy drum head changes! Drummers will love you for having this!
Radial Direct Box—I have two Radial J48 Active DI boxes for various functions around the studio. My primary use is to record direct signals right off of guitars for editing. This saves me tons of time. Radial products are built like tanks and are a million times quieter than just about any other DI box on the market.
Presonus Faderport—You may not need a 96 channel console, but there are some instances where it's hard to get around having at least ONE fader for automating vocals and other parameters inside your DAW. There are a few companies who make quality units, but I think the Faderport fader and USB controller is by far the best. The high quality motorized fader is exactly like the ones on high end DAW controllers and consoles. I've had mine for years now and I swear I'll never go back. The faderport is a great thing to have at your disposal!
Inline Pads—I learned the benefit of keeping inline pads around from Phil Ramone. Phil told me that he kept 3-6 pads in his bag at all times. As a trumpet player I find that I am often overloading preamps and having a -10db inline signal pad is a huge help. Be sure to buy them in pairs for when you're recording stereo instruments!
Headphone Adapters—Nothing shuts down a session faster than not being able to find a headphone adapter. Headphone adapters must have legs because I swear, they're never where you left them and I never have one when I need one... That is until I went on eBay and bought a lot of 50 of them for about $20. Now it's like a headphone adapter infestation.... I'd rather have that than have none at all.
Guitar Picks and strings—Every studio should have extra picks and strings just in case a guitarist is in a bind. If the guitarist breaks a string and doesn't have a backup pair, you get to be the hero when you pull out an extra set. Keep several packs of electric and acoustic guitar strings of varying brands and gauges. A coffee cup full of different kinds of picks for guitar and bass is great too!
This is just the start. The motto is "be prepared for anything" so that when the time comes you're in need, you don't have to go far. Once you've worked with these accessories long enough, you'll not be able to do a session without them. Happy recording!