What's your favorite homemade part or tool?

I do the same with the wires. I use pins from connectors that I cannibalize from old PCB's to solder on the end of the wires....
For the wire, I use christmas light strings.
I have a bunch of other horrible hacks, but I can't think of them right now.
Rich

A piece of springy steel strapping material, folden in half, held closed like tongs, drilled and tapped while closed.
It is used to shorten screws on a grinder. Thread screw through with tongs closed, and the spring tension keeps it from turning while grinding.
I made mine a decade ago, and still have it.

When making multiwire cables from single strands, a great way to twist them is to lay them all parallel, secure one end in a clamp or whatever, secure the other end in a cordless drill, pull the bundle taught, then pull the trigger. Works great and takes almost all the work out of this chore.
-Brian

Here's two more breadboard tricks I use...
How do you connect female header cables into a breadboard? (For example, if use the Atmel STK-500 development board like I do, which uses 10-conductor female-female header cables to connect to the ports, and you want to connect an end of one of these cables to your breadboard.)
If the female header has only 1-5 leads, use a double ended header as an adapter. (Use the type with long leads at BOTH ends - normal ones have a long lead at one end and a short lead at the other, to solder into a pcb board.) You can get these headers in single and double rows. Cut off the number of pins you need, plug it into the female connector and now you have a "male" connector that will easily plug into any breadboard.
For dual-row female connectors with 6 or more leads, I use a dual-row WIRE-WRAPPING male header socket as an adapter. Just bend out the long wire-wrapping pins slightly, to stradle the divider strip of your breadboard, and this simple adapter will let you plug the female-connector cables into your breadboard without any problems.
~WEC

Want a good way to hold solder for those delicate operations?
Get a cheap, clear bodied, non-retractible ballpoint pen (Bic's are perfect) and remove the ink/ballpoint stick. Get some solder and twist it around the ink/ballpoint stick as tight as you can until you have a "spring" of solder about as long as the body of the pen, and cut off the spring with an extra 2 inches of uncoiled solder on one end. Pull this "spring" off the ink/ballpoint stick and feed the uncoiled end into the rear of the pen so that the solder sticks out where the ballpoint used to be. Now you have a great soldering tool. As you use the solder, pull more of it out of the pen from the "spring" within. This is a handy way to hold several inches of solder with minimal skin contact (it -is- mostly lead, after all) and great control for very precise soldering. It also eliminates bulky rolls or excesive waste from lots of "shorts" of solder that were too small to safely hold after using most of it.
~WEC

I built a RC timed pulse generator with some 74AC04 (high rise time). You can adjust the pulse rate and pulse length using two pots. It's good enough that you can use it with an Oscilloscope as a TDR to measure cables (length, breaks, etc...), and it's great for testing inductors/ capacitors
I have been wanting build a bunch of ethernet enabled instruments, DVMM, programmable power supply, etc... In same cases it would be just adding a ethernet->(serial, USB, GPIB) to an existing instrument. Ethernet is nice since more than one computer(PC/laptop) can access any instrument, rather than having to serial/USB based instruments into the right laptop/computer...
I make my own alligator and square pin socket jumper cables. They cheaper to make youself and tend to be more rugged (i.e. soldered in addition to crimp connection to wire).
I have found in many cases making a simple 2 sided PCB and using one of the cheap protoyping services is much faster than hand building a circuit (point to point or wire wrap). Especially, if there are lot of buses, etc... ExpressPCB is perfect for doing these one offs.
-ingo

This is a great thread, I hope everybody gets involved and suggests ideas.
In response to the above, while thats a good idea I've often just twisted the strands together at the end and tinned the last 1/2" with solder. Done carefully the solder will not glob and the wires insert easily into breadboard. Your solution sounds more robust, though.
In addition, for making headers on a breadboard, I found that you can buy a PCB style header and use a pair of plyers to move the plastic part upwards. The result is that enough of the header pin goes down to anchor into the breadboard while enough sticks up the top for the cable to grab onto.
On a drive train I'm assembling right now, I found some bearings at Ace Hardware that were totally unlabeled and thus very cheap. I bought up all of them. I can hold them in place with an appropriately sized U-bolt. Thus they build a very strong axle capable of supporting a good deal of weight.
A new part I'm thinking of (though haven't built yet), is to make T joints for joining threaded dowels at 90 degrees. The plan is to experiment with 1/4" couplers and weldbond. I'll report back later this summer when I try it out. These kinds of items are supposed to be available from Grainger but I've not been able to find them either in the store or the catalog. One old timer at the nearby store said he is sure they used to carry them but couldn't find them in the inventory anymore.
The idea behind this is that the robot platform could be built on a kind of scaffolding. Each platform will be a rectangle cut from acrylic, with mounting holes in each corner. The platforms will stack vertically and will be supported by 1/4" nuts and washers. Therefore if I add a new battery thats taller or shorter to the battery platform, I can adjust the platform height to match it.

I use a short piece of square section aluminium tube, drilled all the way through with 3.5mm and 4.5 mm holes for tapping threads. By inserting a tap through the holes and holding the jig hard against the work piece it keeps the tap square against the work piece as I begin cutting the thread. No more wobbly old tapping efforts.

Why bother with the IC pins, twist the stranded wire end and tin it, this will make it stiff enough to insert into the breadboard

I've done that too, but I've found that the IC pins are a better solution if you are using the wires/parts either as part of a dedicated set of breadboarding tools or as part of a project that requires you to insert and remove the wires from the board a lot. The IC pin gives me a firmer piece to hold when inserting the wire; tinned wire tends to bend where the solder starts, which eventually leads to broken wires and sometimes also includes having to dig broken pieces out of the breadboard hole, which is a MAJOR pain in the ass. Also, I've noticed that the solder on tinned wires eventually strips off, which builds up solder in the breadboard holes, clogging them (plus you have to re-tin the wires).
Tinning the wires works fine in a pinch, but the IC pins work better for long term breadboard work. I think if you give it a try, you'll be glad you did.
~WEC