A hat tip is an acknowledgment to someone who contributed to an online endeavor. It is considered proper netiquette to offer a hat tip to someone who has drawn something to someone else's attention, assisted with the establishment of a website, and so forth. It is given as a gesture of respect and appreciation, and has come to be expected in many settings. Even major media such as mainstream newspapers and television shows give hat tips to their web users on occasion.
The origins of this gesture lie in the era when people used to wear hats. When men spotted each other, they were expected to lift their hats — or to tip their hats — as a nonverbal greeting. The hat tip was a form of polite salute which later entered slang, with people saying "I tip my hat to you" or mentioning that "a tip of the hat is due to ..." when they wished to credit someone or to make a respectful gesture when an actual hat was not available.
Online, people appear to have started using "hat tip," sometimes written simply as HT or h/t, in the early 2000s. A HT often shows up in the context of giving credit for finding something. For example, if a reader draws a blogger's attention to an article or event of interest, the blogger might write it up and say "hat tip to reader X" as a gesture of thanks and appreciation. A HT can also include a link to the reader's own site or profile, which means that it has promotional value as well.
Failure to acknowledge the source of a link or piece of information isn't necessarily wrong, but it is frowned upon in some circles. People understand that the Internet is a very large place, and it's not possible to keep up with everything on it, so even when information is freely publicly available, people may not be able to find it. If someone draws someone's attention to something, giving them a hat tip is a polite way of thanking them. Hat tips also tend to create more of a community spirit, with site users feeling more personally connected and involved when they see one of their fellow users credited for finding something.
Members of a community are often pleased to see themselves mentioned in hat tips. It lets them know that the administrators pay attention to ideas submitted by the users, and in some cases, it may demonstrate that administrators are keeping up with the websites of at least some of their users. Hat tipping is especially common in the blogging community, where linking and networking are used as tools to build blog audiences.