http://www.mapeditor.org/ - everything you'll ever need! :) It's free & cross-platform!
Quoted from the site's list of features:
- General purpose tile map editor with XML-based map format
- Supports orthogonal and isometric maps
- Custom objects can be placed with pixel precision
- Full undo/redo and copy/paste support
- Add custom properties to tiles, layers, objects or the map
- Automatically reloads tilesets when changed externally
- Resize or offset your tile map later as needed
- Efficient tile editing tools like stamp and fill brushes
- Supports input/output plugins to open and save files in custom formats