OpenStreetMap has an API to download data and also to contribute to the database. As a result, many tools are built that use it. Some of these tools (e.g. Mapzen for iPhone) are specifically designed to contribute points of interest. With tools like that one has to be in active mapping mode while using them.
Long before geotagging was officially introduced to Twitter, people were including geographical information into tweets. The simplest approach was inspired by the Twittervision project that encouraged people to insert location information in a format L:latitude,longitude: or L:address:. Also several webapps allowed to insert links to maps, that effectively may be considered as geotagging.
Presently, Twitter allows to geotag tweets natively. This makes it an effective passive mapping tool—One may tweet from places she visits without being in active mapping state. And later, examine her stream of tweets and use the coordinates to actively map the places. One goal of the Tweets about Places project is to help to track places that were worth tweeting about and to make sure that they are mapped in your favorite collaborative mapping projects.
Fellow mappers, let's be innovative and use various ways to contribute to open datastores of geographical information.