How should people spend money to be more productive?

I think psychotherapy is probably making me better at my job, although the impact would vary by person and by therapist.

Of course a good office environment, perhaps including renting office space or a larger apartment where you can have a separate room for quiet work, can be very effective (though also quite expensive!)

The suggestions in the comments on this post might be a helpful starting point, though they generally don't dig into the evidential basis for each suggestion.

• A coworking space ($300/mo), or someplace which isn't your own house/bedroom, to have a better separation of work and life
• A second monitor! Doubling my screen real estate increased my productivity by a (wild guess) 10%, easily an incredible investment. If you have a laptop, I'd recommend this portable second monitor ($250): https://www.amazon.com/gp/aw/d/B087792CQT
• Books! Ramit Sethi has an excellent heuristic goes like "If you're thinking about buying a book, just buy it". Don't worry about finishing (or even starting!) every book you buy, just increase your own access to them to increase your expected number of book content consumed.
• A quality microphone (eg this $100 Blue Yeti mic: https://www.amazon.com/Recording-Streaming-Patterns-Headphone-Adjustable/dp/B00N1YPXW2) if you spend more than a couple hours a week on video calls. I think 5% of why I got promoted at my last job was due to quality communication attributable to me having a streaming-tier microphone. See also: https://www.benkuhn.net/vc/

A lot of these tips are covered in "What are your favourite ways to buy time?".

Employing a personal assistant - I understand that there was a presentation on this at the most recent EA Global conference. Also, there has been a general movement towards both acting as, and employing, personal remote assistants to support professionals with life administration and other more procedural tasks. It would be interesting to see the cost-effectiveness of this intervention. 

My hypothesis is that this would be highly effective for those professions who, a) spend a disproportionate amount of time on administrative tasks, b) the opportunity cost of their time is high, c) they are highly-focused and require deep-work time. 

I paid for a lifetime subscription to Freedom (freedom.to/), an app that blocks certain websites from your phone and computer during pre-set windows. It cost like $60 (one-time cost) and has made an extraordinary difference in my productivity.

Things my office has bought me that are well worth the money/that I will buy for myself in the future if I need to: a mouse/mousepad, a second monitor, and a good (comfortable, height-adjustable) office chair.

If your work involves learning (or reading papers), hire a grad student in the field to tutor you (or just read the paper with you to speed up your understanding). One-on-one tutoring is dramatically more effective than most methods of learning, and you can hire tutors on sites like Wyzant for reasonable prices. Sometimes you'll even find a PhD or other expert.

Two large monitors. I'm not familiar with the research, but I vaguely recall reading something about a relationship between the "real estate" and productivity. I know that I feel so much better with enough monitor space to have 2+ programs open simultaneously, and it makes it much easer to refer to 2+ documents without opening/closing different programs.

Meditation is the thing that keeps me sane and without my sanity I'm not productive. The value of my meditation app (headspace) is immense for me. 

And as mentioned before: big/second monitor

In line with Khorton's answer, life coaching

Especially from someone within Effective Altruism, who understands one's longterm goals, having an individual to sit down with each week and talk through any and all issues is well worth the pay. When coupled with journaling, the issues that would otherwise impact one's EA work can be majorly minimized. 

Even issues that may seem small or insignificant on the surface, if they are recurring, can be extremely taxing on one's sustainable EA motivation. The life coaching widely accessed within EA needs to extend far beyond just direct aspects of the career. 

If you're interested in life coaching on better achieving your goals, romance sustained motivation, etc... feel free to DM me. I'd be happy to chat about options beyond just therapy!

Partner suggests:

• Kitchen timer and Pomodoro technique.
• Miro mindmapping and whiteboard app for collaborating.
• Large format paper and colored markers for analog todo lists and long term calendars

“Big monitors” seems like a big win.

But it somewhat conflicts with being able to change locations, work outside or in your car, etc. I find these are also good ways to stay productive and avoid getting in a rut.

Not sure how to resolve it other than to say “have big monitors in at least one place”.

Focusmate costs just 5 dollars per month, which is low enough to at least try it out for a while!

Beeminder could be worth trying out as well, but I haven't personally used it. Here you pay money if you don't reach your goals.

Increasing you Google Drive/Photos storage is worth the money in my opinion. It's quite cheap and you don't have to worry or think about managing your online storage that much.

For people who travel frequently, a lot of the travel experience can be optimized.

• I haven't tried this but I know others who prefer sitting in the first class coach on trains. It can cost as little as £10 extra for Paddington-Oxford, and it helps them get enough elbow/laptop space to be productive.
• Paying for wifi on flights is such an awesome deal. I can be productive for many hours for just $15-40 depending on the airline and flight duration.
• Optimizing the airport experience is pretty straightforward. In the US, TSA Precheck and Global Entry cost ~$25/year and save you at least 10 minutes (and often an hour or more at peak times) per trip at security and immigration, respectively.
• I also try to book flights on the same airline alliance where practical, which helps me accrue status and use business/first class lines and lounges everywhere. I estimate I save at least an hour per trip through a busy airport as a result, since I can arrive at the airport later, go from check-in to lounge/gate in 5 mins instead of 45, not wait for my boarding group, etc..
• Depending on how much you value your time, it might be worth it to switch cabins on flights. Moving from economy to premium economy / business at check-in often costs £100/£600, respectively, for a 10+ hour flight. I know people who value their time at >£150 per hour, and the productivity gains from sleeping well on their flight would make this tradeoff worth it. You can also use credit card points for this.
• I've tried traveling using running shoes vs other footwear. I estimate I walk at least 20% faster with running shoes on than with boots or walking shoes, based on the number of people I pass up and based on how much I beat Google Maps walking time estimates by.

Pay for all the productivity apps that you're thinking of using. Task managers like Amazing Marvin or note taking apps like Evernote are all slightly better in the paid version, and at least for me it's always been worth it. 

Around the house, spend money on shelving. Having plenty of storage space makes it easier to stay organized. 

Thanks for everyone's suggestions!