Although the NSF won't generally allow you to receive more than 2/9 of your 9 month salary from NSF grants, other federal agencies (ED, DOE, ARO, NRL, USDA, etc.) don't have such a rule, so it is possible to get +3/9 summer salary from such grants.
It's also possible to use a "buy out" to pay part of the 9-month salary out of grants in exchange for a reduction in teaching load.
Beyond that, the regulations forbid taking more than 12 months salary from federal grants. Of course, the institution can give a very successful researcher a pay raise so that they can take more salary from their grants.
Here are some other ways to earn additional income:
Consulting. There are usually restrictions on how much consulting you're allowed to do during the academic year but no such restrictions over the summer. You can charge what the market will pay (e.g. I charge for consulting at an hourly rate that is more than twice the hourly rate that my institution pays me.) However, you'll also be responsible for all of the legal and tax complications that arise (you may need professional liability insurance, you'll have to pay self-employment tax, etc.)
Royalties. If you write a best selling textbook for a lower level service course, the royalties paid to you can be a substantial addition to your faculty salary. However, there's a lot of work in writing a textbook and in most cases, the royalties won't amount to much.
Patents. At most institutions in the US, faculty work under intellectual property agreements that lay out how the profits from a patented invention will be shared. Depending on how generous your institution is, you might be able to earn income from patents.
Start your own business. In some fields, it's possible for faculty to spin their research off into a start-up company. There are usually intellectual property agreements with the university that require ownership and/or profits to be shared with the institution.