If slaving over the Clone tool sounds like too much work, then I've got a fast and simple solution that you might enjoy. The downside? Not only does it require the full Creative Suite edition of Photoshop, but it's also a technique you have to plan for when you originally take the photo.
In a nutshell, Photoshop can take a series of similar photos and automatically remove parts that don't belong in all the photos, leaving behind only those sections that are identical in every one. Here's a crowded shot, for example, with people moving around in the scene.
Here's what to do: When you take a photo of something in which a bunch of tourists are cluttering up the photo, take a bunch of photos so that, over a handful of shots, you thoroughly photograph every bit of the scene. In each photo, you'll have unwanted people, but they'll move around from shot to shot, leaving the background consistent in every frame you shoot. It's best to shoot this on a tripod, but if you're hand-holding the camera it might be okay; just be sure to keep the framing as consistent as possible in every photo.
That's all you need to do. Photoshop will think for a little while (depending upon how many photos you supplied), and you will end up with a photo with no tourists, almost like magic. I took a series of six photos to get this human-free photo.