Please refer to the UMS forum for help next time.
Here are some programs that can do square and/or rectangle terrain:
StarForge
Currently in beta testing stages, StarForge promises to be "the only editor you'll ever need - or want." Not all features are complete, but it already has gained high praise for its terrain editor, ability to place units for 256 distinct players, and the ability to place sprites.
Guedit 3.6 Alpha
This nifty program can do a lot of nice stuff that every map maker needs. It can protect maps, set upgrades to instant, save triggers as text, create square and tweaky terrain, change the map terrain, and change player's colors.(can have all players to be black)
ScmDraft:Alpha
This program is basically an advance Staredit Program. It has features that Sc X-tra Editor doesn't have and doesn't have features that Sc X-tra Editor has. Most people use this program to Stack minerals and other stuff. Another useful feature it has is that it can create 1x1(Square) Terrain. Oh yeah, this ScmDraft:Alpha is under .zip format!
ScmDraft:Beta
This program is almost like ScmDraft:Alpha but it's a little worse. It can still place Square terrain, but it won't show up on the map when you place Square terrain. To make it show up. Go to rectangle terrain and then just place 1 rectangle terrain anywhere. After you do that, it'll show up the square terrain that you have placed. You cannot place regular diamond terrain. If you do, a box will show up and show up numbers. If it does, just hold enter until the boxes don't appear anymore. This program can also let you put map revealers for a certain player all over the map in a few clicks.
There's isom, but I don't suggest using that.