WorldEdit

WorldEdit
WorldEdit is a SMP (And SSP) Bukkit Plugin/Mod that can remove or add blocks in a box, cylinder or sphere. Useful for terraforming and creating large structures.

Selecting Regions
To use WorldEdit, you have a Wooden Axe, (Wand), to edit blocks. Get the wand by saying //wand. A Wooden Axe will appear in your inventory. Note you can change the wand item in a configurations file, only in a server. Left click a block to set the first position (while the wand is in hand), and right click another block to set the second position. You may also use //hpos1 and //hpos2. Those commands will select the block you're pointing at, this is useful if you don't want to fly around to select blocks. Another alternative is //pos1 and //pos2. This will set the position as your current coordinates when you say the command.

Advanced Features
WorldEdit can also do way more complex map changes, like regenerating regions, copy/cut/paste operations and run scripts (needs Rhino to work, otherwise you'll get an error message) written on JavaScript which can make much more complex building, like spirals, mazes and pixel arts based on image files. You can download scripts from the mod/plugin thread or use the built-in scripts.

Use for Griefing
In servers, malicious players can use social engineering to gain OP status and WorldEdit permissions. They delete buildings and even crash (or cause severe lag to) the server if they attempt to change many blocks at a time or by exploding ridiculous amounts of TNT, so be careful who you give WorldEdit/OP permissions or use proper roll-back plugins.

Trivia

 * If you regenerate a selection with lava source blocks, the land will stay lit as if the lava was still there.
 * This seems to work with glowstone too.