The other day, the blower motor of my studio's HVAC system died.
The air handler is a large-volume, low-velocity unit (to minimize noise), and the access panel in the wall wasn't large enough to finesse the oversized blower out. My AC service technician gave up, and he asked me to open up the wall before he would return with a replacement blower motor. I ended up cutting a few strategic holes in the wall, dissembling various portions of the air handler through the holes, and rewiring some AC lines that were in the way. By the time I had successfully extracted the blower, I'd pulled out various hand and power tools, including a number of specialty tools that I've found useful over the years as a studio owner and "de facto first-line" tech.
The Milwaukee Offset Drive Adapter ($20 street) goes into the chuck of your 1/4'' electric/cordless driver (or drill) and angles the drive axis 75°, allowing you to fit the driver sideways into spaces that normally wouldn't accommodate the driver straight (e.g., inside of guitar amp cabinets). Milwaukee also makes a heavier-duty unit with a 90° turn, but I find that the 75° angle allows more clearance for the body of the driver.
I also have a collection of manual right-angle drivers, including the low-profile Neiko 3044A Mini Ratcheting Screwdriver ($9), which is great for light-duty work when there's very little room for maneuvering (e.g., the inside of gear faceplates), and the chunkier (but still compact) Maxcraft 60199 1/4'' Dual-Drive Mini Ratchet Driver ($10), which I pull out when I need more torque. A very cool feature of the Maxcraft is that on the backside of the 1/4'' bit holder is a standard 1/4'' square drive for sockets.