Filet crochet is a fun and simple way to create designs and pictures in crochet--especially in lace. If you enjoy or want to try a filet pattern, but are having trouble finding a design you like, any design expressed as a graph will work.
Fair isle patterns for knitting are a good resource for abstract and geometric designs. Practical Crocheter has done lovely things with argyle patterns!
Bicolor cross stitch charts are another good resource.