I prefer to distribute my projects as .schematics because it makes them easier to import into other worlds, because the file size is much more managable, and because PlanetMinecraft's own Worldviewer applet lets people preview the project before downloading.

.Schematic files require MCEdit to use. MCEdit is - as the name implies - a world (map) editor for Minecraft and is available as a free download from here. Once you have downloaded and installed MCEdit, run the program and point it at your Minecraft folder (if you use the defaults it should find everything automatically). Then click "Import/Export" on the menu, select the .schematic file downloaded from this website, and the project will appear in your world. Use the controls in the editor to move the project to where you want it to appear in your world (when it first imports, it usually ends up hanging up in the sky), save the world file and boom, you are done.

MCEdit is a great tool not only for sharing your projects - .schematic files are a few hundred kilobytes in files compared with the tens to hundreds of megabytes of a world file - but also incredibly useful for building stuff. For instance, in my Medieval Port City project I was able to quickly fill in the streets by importing project files of other structures I had previously built, grade or fill in terrain, or rotate and copy structures.

Different strokes, I suppose. I prefer .SCHEMATIC files myself, so that's my primary way of sharing. It doesn't hurt that PlanetMinecraft. offers built-in schematic hosting whereas I'd have to use a different host to offer compressed world maps. Never underestimate the power of laziness ;-).

More to the point, I have always intended that my builds be used as "plug-ins" for other people to insert into their worlds. Do you need a house or bakery or butcher's yard for your own ? Just download and paste it into your world, no problem. That's why I share them! Admittedly, this city build is a bit large for a drop-in module (but you never know, see comment below...) but that's the philsophy behind all the projects. With a map, you would have to first load up the world and then laboriously cut it out of the otherwise empty world before you could paste it into your own. .SCHEMATIC files are just easier for other builders who want to make use of my projects.

Of course, when I started, PlanetMinecraft's "Schemagic" tool let you automatically view the uploaded schematic file in 3D. It wasn't a very pretty render, but it at least gave you an idea what the project looked like. Unfortunately, the applet is so flakey these days as to be near useless (I get it to work maybe one time out of a hundred, and don't even bother checking most times). Fortunately, Sketchfab has come to the rescue; I've already uploaded a few projects there and have intent on adding the rest. So your desire to view without "installing, pasting, etc." can be satisfied too.

So in the end, I get to be lazy, I make it easier for other builders to use my projects in their own builds, and visitors still can see what the build looks like in 3-D. It's win-win-win! A trifecta of winningness! Hurrah!