How can I rename many files at once?

There are a few but I recommend gprename which is a good compromise between usability and functionality.

Other tools are: rename, krename , pyrenamer , cuteRenamer, ...

I really like qmv from the renameutils package. It enables you to use your favorite (terminal based) text editor to rename files. I prefer to invoke it with -f do which gives you a single column (one row per file) with filenames. Combined with the power of Vim it gives you all the tools you need to do massive filename editing.

http://www.nongnu.org/renameutils/

Métamorphose

https://github.com/metamorphose

may be a little difficult to handle, but really powerful!