The Standard MIDI FIle format is described by the MIDI Manufacturers Association and is available online at
http://www.cs.cmu.edu/~music/cmsip/readings/Standard-MIDI-file-format-updated.pdf

This page also describes it pretty well:
http://www.somascape.org/midi/tech/mfile.html

MusicXML is a standard started by the MakeMusic company (publishers of the Finale notation software), that appears to be pretty well supported and maintained and in use in their products and others’.

Music XML Tablature Format
http://www.musicxml.com/tutorial/tablature/

Guitar Pro is perhaps the best known tablature software.

Guitar Pro 4.06 file format description
http://dguitar.sourceforge.net/GP4format.html

And here’s another tablature software I came across.
TablEdit commercial tablature editor
http://www.tabledit.com/