Pokemon Lake and Pokemon Indigo are the only ones I know of that are even worth your time or even work at all. Pokemon Indigo is having site updates done right now, so it can't be accessed until those are over.
Actually there are five regions the fifth is Isshu from the new Pokemon games Black and White.
Four... Or Five if you include the Champion.
You can find Pokemon from the Heonn Region and after you beat the elite four and go to Professor Birch for your Pokedex update, you can find Pokemon from different regions in the earlier games.
You can found it in the Safari Zone, but to catch it you will have to have passed the elite four and have a Pokemon with rock smash and then smash all the rocks in the Orres Pokemon regions.
Yes you can. Mainly once you beat the elite four, but other more common Pokemon can be found that are not exclusive to the Sinnoh region.
You can ONLY go to the Unova region without cheats and hacks. Although after you beat the Elite Four, new routes are unlocked with Pokemon from all of the other regions, though this is still in Unova.
There are four major land regions in Colorado that include the Rocky Mountains and the Great Plains. The other two land regions are the Intermountain Basin and the Colorado Plateau.
on both Pokemon games to trade from different regions you need to beat the elite four
Seven Island if you don't include the elite four.
An awesome game for Pokemon would be if you started out at kanto like ash then do what he did. Beat gyms, beat the elite four and go on to the next regions
get an egg in one of the regions , orrrrrr... that girl that runs the PC system in hearthome gives it to you after you beat the elite four
the four regions are northeast,south,midwest,and west