If you need the images to be the same width but varying heights (ie not 300 x 300 but 300 x anything) then in settings -> media add 9999 to the height of whichever image size you want to use.

You can also define your own custom thumbnail using the same principle: http://codex.wordpress.org/Function_Reference/add_image_size

If you need the images to be the same width but varying heights (ie not 300 x 300 but 300 x anything) then in settings -> media add 9999 to the height of whichever image size you want to use.

You can also define your own custom thumbnail using the same principle: http://codex.wordpress.org/Function_Reference/add_image_size

[EDIT] As @brasofilo suggests, you can also use 0.