The grandfather of all humanities computing projects (or at least that's what it feels like) and still the XML format of choice for document-centric humanities research.
For anyone working in the Humanities who is seriously interested in creating a semantic represention of a source document (rather than merely a visual representation of it, such as in a word processor or using LaTeX), the depth and flexibility of TEI provides the means to do so. Based around XML, and allowing for self-documentation, this encoding standard (which is supported by the likes of Oxford University, and which is used in, e.g., the Perseus Project and the Newton Project) is likely to be around for a long time, making it an excellent choice not only for the markup features it provides, but also for long-term storage. Although there is a learning curve when approaching TEI for the first time, the inherent flexibility of the underlying XML allows for relatively easy transformation to other formats such as HTML, PDF or MS Word documents, whilst the converse is not true. Indeed it is a splendid project!
What a splendid project this is.