collaboratively underline, highlight, and annotate an electronic text, in addition to providing a mechanism to write additional comments on the margins of the electronic document. For example, Delicious could be used in a course to provide an inexpensive answer to the question of rising course materials costs. RISAL (Repository of Interactive Social Assets for Learning) is another social bookmarking system used for supporting teaching and learning at the university level. Social bookmarking tools have several purposes in an academic setting including: organizing and categorizing web pages for efficient retrieval; keeping tagged pages accessible from any networked computer; sharing needed or desired resources with other users; accessing tagged pages with RSS feeds, cell phones and PDAs for increased mobility; allowing librarians and instructors the capability to follow students' progress; and giving students another way to collaborate with each other and make collective discoveries. One requirement unique to education is that resources often have one URL that describes the resource, with another for the actual learning content. XtLearn.net allows bookmarking of both in one step, the relevant URL being delivered to either tutors or learners, depending on the delivery context. It also demonstrates integration with traditional learning content repositories, such as Jorum, NLN, Intute and TES. Comparison with search engines In comparison to search engines, a social bookmarking system has several advantages over traditional automated resource location and classification software, such as search engine spiders. All tag-based classification of Internet resources (such as web sites) is done by human beings, who understand the content of the resource, as opposed to software, which algorithmically attempts to determine the meaning and quality of a resource. Also, people can find and bookmark web pages that have not yet been noticed or indexed by web spiders. Additionally, a social bookmarking system can rank a resource based on how many times it has been bookmarked by users, which may be a more useful metric for end-users than systems that rank resources based on the number of external links pointing to it. However, both types of ranking are vulnerable to fraud, (see Gaming the system), and both need technical countermeasures to try to deal with this. Abuse Social bookmarking is susceptible to corruption and collusion. Due to its popularity, some have begun to use it as a tool to use along with search engine optimization to make their website more visible. The more often a web page is