Thumbs Plus
Also great for auto renaming, manipulating directories and cataloguing images. Not so great at making thumbs, though.
If you're batch resizing with TP, you can select the height & width to be 500x500, check autocrop, and all the images will have 500px on the long side. You can also overlay any text or image anywhere on the pics.
I've been using this software since the mid 90s. Very simple, fast and convenient.