So, something I do is that I provide web hosting / tech consulting on the side for some friends of mine. Most of them are free, but a few of them are paying clients. Most of the people who do pay do so on a yearly basis, and with my memory, that leads me to often forgetting to invoice them. One client usually pays around tax return time, and just recently had a tech issue that he reached out to me on, and while we were talking he asked if he'd paid this year or not (he hadn't). While looking up whether he had, I realized that there was a whole year that I missed from him, a few years back-- obviously longer than I would like to bring up at this point.

I'm wondering if anyone knows of a simple, easy to use billing system that I could use for my small client base. I don't have the time or inclination to learn something like MS Money or Quickbooks or the like; I don't need anything nearly that complicated. Just something that can kick off on a monthly basis and send out invoices as needed. If I do it right, I can even have it do so for some of my free clients, with a discount applied; that way if they choose to toss me a donation towards hosting costs (as some have asked to do from time to time) they can, but they don't need to since the invoiced sum will be $0. Or one of the clients I have has offered to pay me, but I can't be their full-time on call person (my choice, spoons and all), but then I could invoice them when I do do things for them (updating WordPress and Invision Board mainly).

Operating system doesn't matter; I have Windows, Mac, and Linux available to me. Something web based or designed to run in the background (ie, not relying on me to remember to run it) on a scheduled basis would be preferable. If it can hook into Paypal to actually send the invoices, even better, since that's how I take payments.

Thanks for any suggestions.