Largely, none - most of the books, research material, documentation has been free. MSDN, free, etc.

Time though, that's expensive.

Medium.com and JetBrains Intellij Idea subscriptions.

Best paid investment - my Jetbrains subscription has easily returned many multiples in saved time when working with code.

Best free investment - A tie between MDN, the official Python docs, open source code on github, and youtube. It's pretty ridiculous that one can get a six-figure career just by taking the time to read and understand stuff that's freely available.

Worst "investment " - crypto for sure

I think it was my cats. I am pretty sure they help keep me sane sometimes. Hugely stressful when they're sick and hugely expensive; but they keep me humble, appreciative, and hugging them (_especially_ having them come for hugs on their own) is like an instant shot of happiness.

Your digital cats?

Oops, I guess three cups of coffee yesterday wasn't quite enough...

For digital asset, probably the Bokio accounting software subscription for my limited company.

Not sure if this counts exactly, but also developmental and copy edits of my fiction books. Getting professional edits has made me a much better writer.

Didn't buy it, but Counter Strike Global Offensive Sicker Capsule 2. From 0.03€ to 21.12€... 70400% increase in price... Minus taxes on both sides, but still... Crazy appreciation. Other items as well bought me my Steam Deck, multiplying by various amounts.