Where should I start? [...]
where to start?Building an audience + market validation before building a sideproject are the top starting priority for "a side project that makes money".
Start by building an audience (this can be as simple as interacting with professionals on Twitter and/or their own blogs, or even contributing here on HN!). I won't be able to determine what people want without asking people, and I will save a lot of wasted time by building something that I can guarantee people already want to pay for! For example: I intend to walk around my neighborhood with a survey to gauge interest in localized "technology disaster prevention" (aka initial setup of PC & phone backups with verification and increasingly annoying reminders) as a service. The first sideproject is the hardest because initially the audience is smallest but then can be re-used.
I hesitated to post this because most developers have an "if you build it, they will come" mentality (and a tendency to focus on technology/implementation details that they enjoy) that even I personally have a hard time overcoming myself. However, if the criteria is making money, building an audience is the right first step. Once the bare-minimum MVP functions, marketing makes all the difference on the "that makes money" part (see my list of random books to buy elsewhere in this thread)... and there is no point over-engineering something I can't convince anyone to use!
I realize I'm going out on a limb a bit to say that market validation before each sideproject comes second... I know of one example of someone who has built an audience while publicly initiating sideprojects without thorough market validation (focusing on technology instead -- note that this determination is 100% my own armchair quarterbacking with the benefit of hindsight); this person's projects appear to be faltering because of poor market fit. However, it hasn't stopped many from buying into this person's brand / other projects, and that audience is now following the next project even though initially it appears to be trending toward the same mistake!
PS. As mentioned elsewhere I could shortcut market validation by tracking down commercial products (already being paid for) that are getting a lot of visibility and addressing issues raised in bad reviews; however even when going this route I will still benefit greatly by having an audience to market the replacement.
Edit: switched to first person to preach to myself to get off my butt and start doing something!