Here are the assigned Manufacturer ID numbers (ie, the second byte in a System Exclusive message):
Sequential Circuits  1
Big Briar            2
Octave / Plateau     3
Moog                 4
Passport Designs     5
Lexicon              6
Kurzweil             7
Fender               8
Gulbransen           9
Delta Labs         0x0A
Sound Comp.        0x0B
General Electro    0x0C
Techmar            0x0D
Matthews Research  0x0E
Oberheim           0x10
PAIA               0x11
Simmons            0x12
DigiDesign         0x13
Fairlight          0x14
Peavey             0x1B
JL Cooper          0x15
Lowery             0x16
Lin                0x17
Emu                0x18
Bon Tempi          0x20
S.I.E.L.           0x21
SyntheAxe          0x23
Hohner             0x24
Crumar             0x25
Solton             0x26
Jellinghaus Ms     0x27
CTS                0x28
PPG                0x29
Elka               0x2F
Cheetah            0x36
Waldorf            0x3E
Kawai              0x40
Roland             0x41
Korg               0x42
Yamaha             0x43
Casio              0x44
Akai               0x45

You'll note that we use only one byte for the Manufacturer ID. And since a midi data byte can't be greater than 0x7F, that means we have only 127 IDs to dole out to manufacturers. Well, there are more than 127 manufacturers of MIDI products.

To accomodate a greater range of manufacturer IDs, the MMA decided to reserve a manufacturer ID of 0 for a special purpose. When you see a manufacturer ID of 0, then there will be two more data bytes after this. These two data bytes combine to make the real manufacturer ID. So, some manufacturers have IDs that are 3 bytes, where the first byte is always 0. Using this "trick", the range of unique manufacturer IDs is extended to accomodate over 16,000 MIDI manufacturers.

For example, Microsoft's manufacturer ID consists of the 3 bytes 0x00 0x00 0x41. Note that the first byte is 0 to indicate that the real ID is 0x0041, but is still different than Roland's ID which is only the single byte of 0x41.

A manufacturer must get a registered ID from the MMA if he wants to define his own SysEx messages, or use the following:

Educational Use  0x7D
This ID is for educational or development use only, and should never appear in a commercial design.

On the other hand, it is permissible to use another manufacturer's defined SysEx message(s) in your own products. For example, if the Roland S-770 has a particular SysEx message that you could use verbatim in your own design, you're free to use that message (and therefore the Roland ID in it). But, you're not allowed to transmit a mutated version of any Roland message with a Roland ID. Only Roland can develop new messages that contain a Roland ID.