- Don't, it's free
- Allow download only after purchase
- Give away a limited free trial
- Give away an "ad-supported" version that uses up your bandwidth and screen real-estate and, sometimes, time, by displaying ads but is otherwise fully functional.
- The "freemium" model: A limited free version missing functions that are activated after payment, but which is otherwise fully functional and time-unlimited.
Having a limited trial can take one of several models. One of the world's best twitter clients, Gravity, has a 10-day trial during which absolutely everything works, then it refuses to work at all until you activate it. You don't have to reinstall anything, it just checks on start-up if it's registered or not and stops working until you register your phone during check-out after payment.
For networked applications like the Twitter (and Facebook and FourSquare and ReadItLater and Google Reader and Flickr) client Gravity that's not a problem, but unfortunately some applications that don't otherwise need networking at all will be ad-supported or go away and talk to their home server anyway, which is a nuisance and potentially expensive.
Another popular model is to allow the software to run indefinitely, but not to save anything. I've seen this model used in 3D modelers that can load or edit anything, but can't save any changes.
Or, sometimes, the "limit" is that starting up takes longer and gives you an annoying note, every time, that this wouldn't be happening if you manned up and shelled out for the licence fee.
All of these are, I have no doubt, legitimate. The freemium model is gambling on enough people wanting the extra features, the time-limited feature-full trial offers potential buyers a vital chance to make a complete evaluation, and the pop-up warning and ad-supported models pit the non-buyer's patience against their frugality.
The feature-limited trial can be highly annoying and limit your ability to truly evaluate the functionality, but hey, it's an option.
The only model I really don't like is the "buy it or nothing" option, where you have to take it on faith you will like it and even be able to get along with it.
However, there are a right way and a wrong way to approach each of these models.
The first right way is to put comprehensive and clear screenshots on your web site, regardless of what you're doing with the software. The second right way is to clearly announce what you are doing.
I have just found a piece of software that violates that second principle, and almost as a matter of principle I won't be buying.
It's not from a company and there are better ways to provide feedback, so I'm not going to name and shame here.
I'll just make it really obvious and say I was looking for a small application to track expenses on my N900.
I found three - one free, one freemium and ... another one.
Its website has a huge list of features, mentions it's cross-platform, has screenshots and a video, and it's only when you get to the Download page that you are told, in fairly poor English, that it is fully functional except it won't save data between sessions until you enter a registration key you can get after donating via PayPal.
No price, just making a donation at all.
Now, that right there annoys me just a little. Either decide what it's worth, or ask for donations but don't require them, or just give it away. What, you want the processing fees on a 1c donation? That's what I'm tempted to do, here.
What's worse, however, is that the application is available in the repositories for the N900, so it's possible to find it and install it via the on-device application manager before finding out this limitation. In fact, it's possible to install it, run it, enter lots of data, close it down and then, next time you start it up, find all your data missing, think it's horribly buggy and abandon it in disgust.
The first version I installed was from the extras-testing software repository and it was not, to put it mildly, too advanced. I then updated the entire device (I love a system that lets you do that. As opposed to Nokia's other smartphone OS, that doesn't) and got a new version, with extra features, from extras-devel which, explicitly, is not expected to be safe for regular usage because it's cutting-edge.
So the version that looks complete and usable isn't available to sensible people.
I also noticed a brief notification saying I hadn't registered it, and another one saying it wouldn't work fully until I did.
First of all, I don't remember that second, fairly important, notification from the earlier one, which had the same limitation. Secondly, those notifications fade and disappear fairly quickly. So if you're not looking at the screen, you don't notice it.
It's really, really poorly done.
I shall keep my opinion of the icons to myself.
If you are looking for an expenses tracker for Maemo 5 on the N900, I recommend Toshl. It's freemium, cleanly and attractively designed, easy to use and to comprehend and has full backup onto the web site, where the premium version gives you extra features. It's also available on all the major platforms except WebOS and Windows Phone, although there is a non-functioning button for that so it'll probably be soon.