Sam Altman's Loopt provides location-based functions for cell phones, allowing users to see where their friends are on a map. The software, which uses GPS and other network technology, also sends text alerts when friends come in close proximity to one another. There are also a variety of search functions, such as restaurants near your current location or places your friends have tagged as being favorites. Loopt is currently available on Boost Mobile and is expected to launch on Sprint and other carriers within months.