In a word: ouch. Kickstarter and Indiegogo send 1099s. Gumroad doesn't, but between this and my stock market gains I'm in a really nasty tax bracket, so I set a bunch of it aside for taxes. Luckily? I usually spend a lot of the money I earn from side projects on funding other failed side projects, but I basically plan on giving up 30-40% of side project earnings for taxes.

There is some fancy stuff you can do that helps, but once you get above a certain income level you just have to pay.