Text editors

Any text editor can be used for SVG source files. It’s plain XML, nothing else. Still, some editors are better than others.

Emacs

GNU Emacs has very good support for SVG and it allows you to switch between text mode and image mode.

Inkscape

Inkscape has a built-in XML tree viewer and editor.

Other editors

Lots of editors have XML editing capabilities, like tree-viewing and source code highlighting.