This is wonderfully encouraging. I often find myself talking with people about their ideas. The idea could be for a company, for an app, for a new site, for a side project, but my usual response is something along the lines of “go for it!”
Take that, of course, with a grain of salt. If you’re talking about starting a company and devoting 100% of your available resources to it, that’s a very serious consideration that deserves a lot more thought than a buddy telling you to go for it.
Side projects, site ideas, etc. are a totally different story though. Your idea sucks, now go do it anyway speaks to the fact that many times when we really pursue a idea, it rarely comes out exactly as we had envisioned and more often than not comes out better. Ideas are stellar in our own mind, and sometimes it works out just like that and everyone else suffers from the same problem. Basecamp comes to mind, but not a whole lot else to that degree. Very often your idea finds common grounds with a large number of people, but I see the value in receiving and evaluating outside input for the better of the idea itself.
That said, pursue your ideas. Who cares if it’s been done before? At the very least you’ll create more experience for yourself, and you can never have too much of that. Check out Your idea sucks, now go do it anyway, it’s got a number of examples outlining services you likely use, and what the original idea was.