Is a “donate” button effective?
What about AdSense?
Any thoughts on Micropayments?
I'm in the process of writing a guide to website monetization now, but obviously that doesnt actually help at this point in time.
First off, how much traffic do you have. Any less than 1000 uniques a day and you are totally wasting your time even considering monetization.
Donation buttons are useless unless you have compelling content that people really appreciate. Tell us what your web app or URL is, and I'll give a better judgement.
Adsense is reliable, but wont pay much unless you have niche ads and a niche audience clicking on expensive ads. Again, you would need at least 1000 uniques a day before you even started seeing a $50/month cheque from adsense.
Micropayment you would need to define better, I guess it depends on the web app. Generally though, I dont think users would appreciate suddenly having to pay even a small amount for something that was once free, especially if they had to load up their account for just one transaction.
Personally, I've had the most success with buysellads. I get about $250 to $500 by selling ads direct to advertisers with their system - chargin about $50 per ad slot on my site which recieves 100,000 impressions a month.
In general all are viable options while none really guarantees to monetize your application. It all depends on the user base and if they are willing to support you.
Even if you have 10.000 regular users who are more than pleased with your web application you can't say they are going to donate ~ if I had to think of numbers I would say one in every 1000 users is willing to donate $5 and that's a one-time payment, not a monthly income.
AdSense very much depends on the daily page views, the "Click Through Rate" (percentage of clicks per impression) and the "Cost per Click" value of the shown ads. There are various AdSense calculators out there if you are interested but as long as you haven't used AdSense on your page before the actual "Click Through Rate" is a guesswork and clicks can be worth anywhere from $0.01 up to $6.
Last but not least keep in mind that
(a) advertisement can kill your users experience
(b) AdBlocker (and similar) is the number one extension/ad-on for pretty much all browsers out there
I would say with 1000 daily visitors the page will somewhat pay for itself (for the hosting).