(EDIT 14/07/08: Kristin pointed out in the comments that I didn't say what yarn/needles I used. I've added that in now. D'oh!)
Here's a pattern I worked up a little while ago. I dug it out and knitted one up for my giveaway last week, so I can call this tested now, so here it is. It's a very simple lace bookmark and I got the lace pattern out of a stitch dictionary.
Fern Lace Bookmark
Yarn: I used Sirdar Country Style 4-ply, which is a wool/acrylic blend. I've also made one in Wendy 4-ply 100% cotton. So any 4-ply (fingering-weight) yarn would do, though preferably not too fluffy, or it won't show the stitch pattern.
Needles: I used 2.5mm needles, the same ones I use for socks.
P - purl
K - knit
k2tog - knit two together
yo - yarn over
sl1 - slip one stitch knitwise
psso - pass slipped stitch over
ssk - slip two stitches knitwise, slip them back to the lefthand needle, and knit two together
1 (and all odd numbered rows): P
2: K1, p1, k2tog, k2, yo, k1, yo, k2, sl1, k1, psso, p1, k1
4: K1, p1, k2tog, k1, yo, k3, yo, k1, sl1, k1, psso, p1, k1
6: K1, p1, k2tog, yo, k5, yo, sl1, k1, psso, p1, k1
Repeat these six rows to form the fern lace pattern.
The bookmark is worked in three sections.
Cast on 13 stitches
2: K1, p11, k1
Work the six rows of the fern pattern, and repeat them until the bookmark is the length you want.
2: Ssk, p1, k until 3 stitches remain, p1, k2tog
Repeat rows 1 and 2 three more times. Five stitches left
3: Ssk, p1, k2tog
5: sl1, k2tog, psso
I don't have much experience writing up patterns, so let me know if anything doesn't make sense!
PS: For those on Ravelry, you can find the pattern here. Please put it up if you make one, I'd love to see them!