H&R Block
H&R Block is a long-standing tax preparation firm, and they also offer a program for doing U.S. federal and state income taxes yourself. This program was formerly called TaxCut, and is now H&R Block at Home. Its file format changes every year (as is normal for tax programs keeping up with tax laws/regulations/forms which also change every year), and uses a file extension of .t followed by the last two digits of the year.
Each year's program can only open that year's files directly, but they can also import data from earlier years' files in order to carry over figures as appropriate. (And they can import past-year data from the competing TurboTax program as well.)
The format is a proprietary binary format. Bytes 05 and 06 (where the first byte is 00) appear to contain the year the tax return is for as a 16-bit little-endian integer.
