Ok, there are really three or four who deserve special mention, but I'm only going to focus on one at a time. Remember, your attention span is shrinking as you read this :-) With the single exception of Apple Computer, more of the software on my MessagePad has come from Stand Alone, Inc. (SAI) <http://www.standalone.com/> than from any other vendor. For that reason alone I decided to pick on them first.
According to their web page, SAI was formed in 1995 "with the purpose of creating a line of Newton Software which would showcase the platform." Well, I'm here to tell you they've accomplished that goal nicely. As of this writing (Halloween, 1997, for no particular reason) they list more than 50 packages on their online order form. Even more appear in a list of demo packages available, so who knows what the real count is?
Arguably, the most "famous" of their products is Backdrop+, a very nice backdrop application -- every newton needs one, and the less heap (memory) it takes up the better. Backdrop+ takes up very little heap and provides some very nice features. Plus, it just looks cool. That was, I think, the first package I purchased from SAI. On the other hand, my first purchase might have been Super Caps, a great little utility for changing the case of a whole line of text with just a single tap. It's handy because I have rather sloppy handwriting and the handwriting recognition engine sometimes takes me a bit too literally.
The most recent product I've purchased, just today as a matter of fact, is StartBar. StartBar is a Windows95-like taskbar useful for switching between running applications. It seems rather superfluous since starting Newton applications is so quick (on the MP2000 and later machines, anyway), but it is surprisingly handy and fairly unobtrusive. This is indicative of several SAI software packages -- small utilities that make using the Newton even easier than before.
In addition to the packages I've already mentioned, I also use their freeware Freeze Utilities, useful for activating the hidden "package freezing" feature of the Newton OS, and I have the free game Black Jack. So, not only do they provide useful programs at reasonable cost, they also have some freebies, I guess as a gesture of good will.
In addition to supplying great software, I also want to briefly mention that dealing with SAI has always been a very positive experience. I can't say much about how they respond to bugs -- I've only had to report one; it was StartBar when it first came out. I don't recall if it was beta or version 1.0, but that doesn't really matter. I reported the bug on a Thursday and received a response and a new copy of StartBar in the mail by Sunday. That's service!
Finally, their web site. It's fantastic. It may never win one of the "best of the internet" awards, but it ought to. It is easy to navigate and provides precisely everything you need to make a purchasing decision. Each product has its own page complete with screen shots and a clear description of how the program works or what it does. Their site also provides an easy way to download evaluation copies of all of their software, and has a convenient online order form for those who like to shop while they surf.
Hats off to Stand Alone, Inc.