On PCs, the key labelled `alt' is usally mapped to `meta' by your X server. Thus, M-x can be typed with Alt-X. Of course, if you want to bind A-foo in Emacs, that is a problem. You can, if you try hard enough, manage to map Shift, Control, Meta, Alt, Super, and Hyper with a PC keyboard. Then you can bind your favourite commands to such key combinations as C-M-S-A-H-!.
Of course, if you have a Sun type 4 or type 5, you are much better off as far as modifier keys go.