Although MIDI is a widely accepted, known and used protocol in the music industry since almost 40 years, in 2014 I can recommend only one (!) standalone software for a PC running Win 7 64-bit that was exclusively designed for manipulating MIDI data. It is MidiEditor by German programmer Markus Schwenk, which was first released in 2010. To put it briefly: It is brillant, and it's for free :-)
... by Laurie Spiegel (Shareware for Mac/Atari/Amiga computers). One can use Music Mouse intuitively without any specialist knowledge except how to move a computer mouse around the screen. The musical output always sounds "interesting", but if you don't work on it really hard, you will get bored very soon. The diffculty here is not to produce musical structures that make "sense", but to compose something out of all the material the mouse is "suggesting". You can run Music Mouse on a Windows PC using an ATARI emulator.
In 2005, I did some music for Player Piano with Music Mouse, e. g. "Three Movements".
... by Lars Kindermann (Freeware for Win 3.11 and higher) is an easy-to-use piece of software. Its structure-generating capabilities are very limited, but this didn't bother me because of the playful concept. Good for beginners in algorithmic composition.
The electronica compositions "ambient 01" to "ambient 10" were created in 2000 with the help of MusiNum.
... by Russian software company NTONYX (Shareware for Win 98 SE and higher) has an original approach to generating music: It processes a given SMF by transforming its metric, rhyhtmic and harmonic structure, thus outputting a new SMF. NTONYX says nothing about the underlying heuristic (?) algorithms, but my experience with the program's output indicates they may be not that ingenious. Especially when the input SMF is rather complex, Style Morpher simplifies its rhythmic and harmonic content in a quite predictable way. But may be you have to go the other way round...
Style Morpher was very important in composing the "Piano-Rag Morph" after Strawinski in 2007 and the "Morphe héroïque" after Satie in 2008. It was also used in creating my "Sonata 2006" in 2014.
... by Paul Whalley (Shareware for Win 95 and higher). Be patient! It takes some time to get something interesting out of it, but once you got the concept, its capability of creating melodic variations on a user-given scale is impressing. Unfortunately, Paul's homepage disappeared recently, so you have to google for the software.
I used Tangent for composing sets 1, 2 and 5 of my "Septet" in 2001 and parts of "untitled [ambient 11]" in 2003.
In 1992, programmer Randy Stack had the idea to develop a Text-to-MIDI conversion tool. "ASCIIMID takes any DOS text file and outputs a corresponding Standard MIDI File Format 0, according to the following rules:
You can download ASCIIMID here, including Randy's original ReadMe. The software doesn't run under Win 7 64-bit, so you have to use a DOS emulator.
In 2006, I used ASCIIMID for processing a text of Austrian writer Thomas Bernhard, to help composing "Bernhard" for orchestra.
... by Brad Smith (Freeware) "allows you to alter the tuning of your MIDI device. [...] It works by sending out each note to its own MIDI channel, where a pitch bend may alter the pitch as per the tuning you specify." This is a simple to use software to implement microtonality in realtime to existing SMFs. The SMF itself remains unchanged by intun, but of course you can record the altered output. For me, intun also worked fine in a live peformance. Use LoopBe1 to feed it some MIDI data.
Intun was used to re-tune my "Sonata 2006" in 2014.
...is "an internal MIDI device for transferring MIDI data between computer programs", as programmer Daniel Schmitt puts it. You can also call it a very flexible, stable and easy to install relay to interchange MIDI data in realtime between several applications running on your computer. It's for free.
Piet van Oostrum's small, but brilliant (and free) DOS utility converts SMFs to ASCII-Text and vice versa. You can edit the ASCIIfied SMF easily in any text editor, e. g. Notepad. Here you can find a version of the software that runs under Win 7 64-bit. (look for "Program / Contents / Text To MIDI")
The free Virtual MIDI Piano Keyboard by Pedro Lopez-Cabanillas displays a very big and highly customizable piano display on your desktop. Use LoopBe1 to feed it some MIDI data.
Despite its flashy name this is a rock-solid piece of software by German programmer Daniel Pustka. Apart from playing Standard MIDI Files without any timing problems, you can build playlists. And it's free.