While the lifeblood of a commons is commoning, many types of commons nonetheless need sources of capital and credit, if only because they exist within capitalist societies operating within conventional banking and finance systems. Investment and credit can be important to many commons, especially those requiring infrastructures and interactions with markets. However, given the known dangers of extraction, exploitation and social disruption associated with capitalist banking and finance, capital and credit within commons must be deployed with strict systems of accountability and mutualized benefit. Attention must also be paid to maintaining the integrity of community relationships amidst the socially distorting influences of money-flows.
The use of capital cannot be allowed to metastasize into capitalism, at a minimum, and debt cannot be allowed to become a new instrument of structural domination and control. For this to occur, new social circuits for the flows of money need to be created so that a new type of system can integrate the social and the financial into a new socio-economic paradigm.
Fortunately, there is an eclectic array of alternative money, banking and finance systems that may be adapted to support the development of commons. These include community development finance institutions, social and ethical lending banks, public banks, transition-oriented credit experiments, complementary currencies, blockchain technology (adapted from Bitcoin) to enable co-operative finance, crowdfunding support of projects, commons-based virtual banking as exemplified by Enspiral, and cooperatively managed value-chains for provisioning such as ones developed by the Solidarity Economy movement and Food Commons Fresno html 
(California).
With varying degrees of success (because they are embedded within larger capitalist structures), these projects are capable of providing capital, credit or new types of currencies to foster commons-based provisioning -- while trying to escape the familiar traps of capitalist lending and investment.
In addition, there is an important monetary reform proposal that seeks to reclaim the state's right of seignorage -- the right to create money -- from private banks. The idea, as outlined in rigorous detail by British monetary expert Mary Mellor, argues that the state can create money for all sorts of ecological and social purposes -- "quantitative easing for people" -- without it being counted as public (government) debt. This is precisely what occurred during the 2008-9 financial crisis except that the new money creation was used to bailout insolvent banks. Mellor proposes new ways to manage money-creation so that the benefits of money-flows can accrue to taxpayers and commoners.
# Sources
David Bollier and Pat Conaty, "Democratic Money and Capital for the Commons: Strategies for Transforming Neoliberal Finance Through Commons-Based Alternatives" (January 2016).pdf
Mary Mellor, Debt or Democracy: Public Money for Sustainability and Social Justice (Pluto Press/U. of Chicago Press, 2015). blogpost