On playing -- make that plying -- with thread :: this is a great way to ensure you keep the length and width of your singles in your plied yarn! Thread doesn't add substantially to the finished yarn diameter.
You can get some neat techniques with thread. You'll get a bouncy, faux-boucle effect if you hold the thread in one hand, tight, and the yarn in the other hand, not quite as tight. The thread will "cut" into the yarn a little bit and the yarn will tend to "bobble" as it is plied with the thread.
Even more fun is a coil yarn -- hold the thread taut, straight out from the bobbin; hold the yarn at 90 degrees (a right angle) to the thread and let the yarn completely wrap around the thread, forming coils. For this, you want your singles to be fairly high twist, so they hang together well with this amount of "plying" twist.
Knots is another coil variant, where you actually put coils on top of each other for about 1", back and forth, to make a big lump. (These aren't actually knots, just big layered lumps of coil.)
It's fun, too, to combine these two techniques so you alternate between bobbly and coily. Both will cut into the length of the single, since you aren't holding the single straight out against the thread, but are encouraging it to scrunch up a little in the bobbles, and alot in the coils.
If you just want a straight ply (also useful), but aren't sure of the amount of twist you need, take a 4" length your single that has had the twist set in it, a 4" length of the thread, knot them together at one end, and put them in a sink of warm water. "Milk" the yarn/thread strands so that as the yarn gyrates, it wraps around the thread. The warm water "activates" the twist in the yarn, and you need it to twist with the thread to show you how much plying twist this yarn wants to have a balanced ply with the thread. Save this sample and compare with it as you spin for a balanced ply (if/when you want such a thing).
Plying with metallic threads is very fun looking and really spices up a single. Even fine metallic threads add glitter and glam to the yarn.
Dana asked me what sort of metallic thread I ply with.
I have to admit, I'm often not too picky about my threads. I found some neat/cost efficient metallic thread in Joanne's next to some very short length bobbins of metallic thread -- the "big" bobbin was cheaper per bobbin than the "small" bobbin, so be sure to check out yardage on the bobbin. Most of the metallics seem to be polyester, though if you head over to the thread display you may find rayon and nylon in metallics as well.
I'd tend to pick out a round thread over a flat one, just for reducing irritation caused by flat metallic threads/tapes.
For fancy, a sewing machine specialty shop may have silk threads -- I'm not sure if Joanne's has that level of thread or not.
(based on posts by me to Spinning_on_the_edge, 3/2007)