Anything that increases team productivity significantly. If my team of developers is > 10 then I'm probably spending about 100k per year in supporting tools:

- source code management - server monitoring and metrics - payment processing - testing tools - continuous integration tools

Any of those could easily cost 10K/year, or more, depending on what the team is doing and how fast they are growing.

Beyond that, anything that ties back to revenue. Email marketing, PPC, SEO. Also possibly things that optimize finances, analytics or anti-fraud.