In the International Phonetic Alphabet (IPA), the tilde is used to mark nasalization, and is placed above any phone that is nasalized.
In Japanese, this symbol is used to indicate ranges. 12 ~ 15 means "12 to 15", ~ 3 means "up to three" and 100 ~ means "100 and greater".
In English, it is often used to mean "approximately." Therefore, ~10 would be "about 10." Similar symbols are used in mathematics, such as in π ≈ 3.14, "pi is about equal to 3.14."
Used in URLs on the World Wide Web, it often denotes a personal web page or web site which resides on the website of another company or organization. For example http://www.widgets.com/~johndoe/ might be the personal web site of John Doe, on the website of the Widgets company. This comes from the Unix shell usage of ~ followed by a username to mean the user's home directory. However, when accessed from the web, file access is usually directed to a subdirectory in the user's home directory (often called public_html or www).