Passive Income the easy way: Invest money in an S&P 500 Index fund. Routine dividends are easily count as passive income. You need a lot invested for this to be a significant amount, though.
The reason this may be easier for a web developer than, say, a retail cashier is that the web developer most likely has a higher salary.
--- Owning a company, whether services company or SaaS is also another way to get passive income. However, unless you have huge capitol to hire folks to do the work, or put a lot a time in to build the company [and then replace yourself] it may not be practical; and it definitely isn't immediate.
--
I've done the writing route, writing, and self-publishing, technical programming books. It is a great way for passive income. Once it is published you get residual income each month.
Much to my surprise, I've found that programming books have a much longer tail than I would have expected.