Washcloths
A washcloth is one of the most basic ways to use up a small amount of yarn. The Washcloth Quartet and the Leafy Washcloth uses about 40 yards of yarn per minicloth. Another great thing about knitted washcloth patterns is that they are easily modified to make knitted towels or scarves.
Hats
Who doesn't love hats? They don't take much yarn, they're quick to knit, and they're fun to wear. Still have an extra skein? Knit yourself a pair of matching mitts. Check out the patterns for hats and mitts.
Bags
Small knitted bags, pouches, phone cases are another way to use those single skeins. Here's a pretty Lace Bag from Tsuki's Knitting. And you can try out Margalo, a pattern for a miniature gift bag right here on Tricksy Knitter.
Fingerless Mitts and Cuffs
The Montgomery Mitts fingerless mitts pattern uses under 136 yds of yarn, and is a nice quick knit. In the Idiot's Guide to Knitting, you'll find patterns for Fair Isle Fingerless Mitts as well as easy Buttoned Wristlets. And, although it's not quite a one skein pattern, Eunny Jang's Endpaper Mitts are none the less a perfect use of a small amount of 2 color of yarn. And so pretty.
Cowls and Shawls
A cowl is a great one skein knitting project. It has the warmth of a scarf, makes a great gift, but takes much less yarn than a scarf. The Bittersweet Cowl, worked in the smaller size, uses about 200 yards of yarn, which is about a 100gm skein of worsted weight yarn. Looking for something alittle more elegant? Try a shawl using a single skein of hand-dyed silk cashmere yarn.
Toys
Toys and animals make great one skein knitting patterns. Check out Rebecca Danger's site for some truly whimsical robots, bunny nuggets, and other creatures. And here's another favorite of mine, Hansi Singh, author of Amigurumi Knits.
One skein books
Not to toot my own horn, but the Idiot's Guides: Knitting has several excellent single skein and scrap yarn projects.