Very closely related to the previous example is the obvious case of Microsoft's Windows OS and the thousands of applications that only run on Windows and nothing else. It's different from anticompetitive partnering only because there is no formalized relationship, other than the developer-vendor one. More developers choose Windows-only because it's cheapest and easiest to focus only on the dominant player. But it's by no means confined to Microsoft and OSes. In fact, it's one of the most widespread practices in the industry. Any maker of software that encourages other developers to make dependent applications is hoping to lock-in some customers that way.
Peripheral Availability and Drivers
One of the major challenges that an emerging operating system faces is that of supporting the various peripherals and accessories that a user might have. Virtually every peripheral in existence supports Windows, and though some commoditized peripherals like keyboards might be easily supported in an alternative platform, others, like video cards and printers might be very difficult, especially when they contain special features that require proprietary information from the vendor. Even widely-used platforms like Linux and MacOS suffer from this problem, to say nothing of a marginal platform like SkyOS or AROS.
There is very little that an emerging platform can do about this. Solutions include: 1) begging manufacturers to write drivers for your platform; 2) trying to write them yourself, through various methods of hacker heroism and brute force; 3) focusing on a small subset of peripherals, perhaps even manufacturing them yourself, and advising your users to use only those.
Some non-OS platforms have this sort of lock-in. Specialized applications in scientific, musical, or other niche fields have hardware devices that work exclusively with particular software.
A closely-related method of lock-in is also probably the oldest. The earliest computers were all hardware-software combos. In fact, as many people have pointed out, the "sharing culture" that was prevalent in the software world before software was widely commercialized emerged because hardware vendors made their money from hardware, and the software was seen as almost incidental. But hardware vendors locked their users into their software because you needed their software to run their hardware. It wasn't until IBM created the "open" PC platform that it became feasible to write very low-level software for a platform without the say-so from the hardware vendor.
Apple is of course the most notorious practitioner of this method in today's consumer computing world. If you want to run the Mac OS, you buy a computer from Apple. Even their imminent move to the x86 platform won't change that.
Look and Feel
As people become accustomed to the way a particular tool works, that familiarity can sometimes act as a disincentive to switching. For example, I'm used to driving on the right side of the road. When I went to South Africa a few years ago, I had a fun time driving on the left. I was even able to become accustomed to shifting with my left hand. Luckily, the clutch was still on the left, or I would have been in big trouble. For some reason, though, in the car I was driving, the turn signal lever was on the "wrong" side, so whenever I would make a turn, I'd end up turning the windshield wipers on. Even after I was well-accustomed to driving on the "wrong" side of the road, and using the "wrong" hand to shift with, I was never able to kick the habit of using the windshield wiper lever to signal turns. It was just too ingrained.
Similarly, people have become accustomed to the way their computer's user interface looks and works, and switching to another platform can be frustrating. Mac users trying Windows, and vice versa, can be an initial challenge, despite their similarities. Microsoft knew this. When the first versions of Windows came out, and were a blatant copy of the Mac's UI, Apple sued, unsuccessfully, setting a precedent for the un-protectability of look and feel. As a consequence, many tools for Linux adopt the look and feel of Windows, or of common Windows applications. Many true believers and UI snobs decry this, but there's a sound reason behind it -- lowering the barrier to entry.
There's an interesting trend in the computing world that serves to undermine vendor lock-in, but also enables some scary new forms of potential lock-in that make all these other ones look like child's play. The internet opened up the possibility of applications that run on a server far away but can be accessed by a user over the internet, through a standard web browser. As I mentioned earlier, Netscape saw this new reality as a way of lessening the importance of the operating system for much of everyday computing, and so did Microsoft. If we look at today's world, much of what a computer user would have used a standalone application for just a few years ago is now routinely, or even exclusively done online: looking up a word in a dictionary or thesaurus, finding a map, reading an encyclopedia, browsing a magazine's archive, even everyday email use, managing photos, keeping a journal, updating an address book, scheduling.
In fact, almost all of what people regularly use a computer for can be done today online with hosted applications, even word processing and photo retouching. Some online applications, like Salesforce.com, have become fantastically successful commercially, with companies paying relatively hefty per-seat fees, because they're so good, without all the management hassles of traditional software.
The scary part is that with the vendor having absolute and total control over the application, its features, and even your access to it, they're in a position to keep you locked in like never before. They even have custody of all of your data in most cases, and in some cases, they have the only copy. Only time will tell what impact this trend will have on the individuals and businesses that use these services.