I have written a few programs that are being run over 10,000 times per month. In order for people to use them they have to be a member of my forum (There are around 2,300 members). I know that there is a lot of potential in it so what would be the best way to monetize the programs without actually forcing users to pay money for them?

That's a great question. I saw something recently at SEOMoz.org which you might apply. When you run a PageStrength of keyword difficulty repot (can't remember which), an ad shows in the browser, which will open a new window if clicked on. You might do the same while the download page is opening...

Use a donation button...

I think I'll go with the less obtrusive approach (the donation button) which will lead to a page with affiliate offers the people can sign up at. They are greatful enough for the programs I make them. :P

Can you not insert an ad into the application itself and sell that out privately? You could make it pretty subtle and still convince advertisers to invest in it I think.

YOu can try placing a coregistration path. So, when the user clicks to use your program, they are shown a few offers to which they can subscribe to. If they click yes, you get paid, if no, they are taken to your product page. -so no obligation.

If you do decide to place offers on the donation page, thats an excellent idea as well. Make sure these offers are Incentivizable though. If you cant find such offers, hit me up and I'll hook you up.