This is a very decent tilemap editor:

http://www.mapeditor.org/

If you dont want to use a external editor, you can simply write your own.

In Javascript this is a piece of cake, especially with HTML5 and Canvas.

- Just define a tilemap array, a tilesize and a way to store mouse position and button states.

- Add a event listener for mousemove, mousedown, mouseup and convert the client coordinates into screen coordinates and store the button states respectively. (You need to take the canvas scale into account!):

var mouseX = e.clientX * (canvas.width / canvas.offsetWidth);
var mouseY = e.clientY * (canvas.height / canvas.offsetHeight);

- In your render loop, you convert the mouse coordinates into world coordinates and calculate the tile position for the current mouse position (Its a simple divide by the tile size) and draw rectangle by the tile position multiplied by the tile size (You may need to take camera translation and scale into account as well)

- At this point you have all tools you need to make your own tilemap editor.

You need some reference, here is one:

http://root.xenorate.com/final/jsstuff/platformer/demos/realcollisions.html