College students have a new alternative to shelling out hundreds of dollars for textbooks, then getting little in return when they resell them at the end of the semester: renting. Textbookflix.com lets online users rent books for a fraction of each book's retail price. The iconic "Physical Chemistry: A Molecular Approach," for example, which usually costs about $100, rents for $39 (plus shipping) a semester. The company keeps up with quick-changing inventories by partnering with Amazon.com and other book-swapping partners, which provide more than 2 million books for rent. Students who need an extended rental or just a quick research-paper reference can find flexibility at BookRenter.com, which has rental periods ranging from one to four months. Users return the book with a prepaid shipping label on time, or the book becomes theirs with an automatic payment of the remaining retail value. Some things to remember: both services allow minor markings, such as writing and highlighting, but encourage using pencils and erasable highlighters. But no dog-earing or tearing out pages. That'll void the rental agreement, and turn your new possession into a lesson in poor investing.