I have realized ERC supports control characters to emphasize text as bold, underline… However, I prefer *foo*, /bar/ and _foobar_ syntax, just like org-mode and Gnus. So I have added the following code to my .emacs to do the job:

(defun erc-emphasize ()
  (when (eq erc-interpret-controls-p t)
    (goto-char (point-min))
    (while (re-search-forward "\\(?:^\\|\\s-\\)\\(\\*\\([^*]+\\)\\*\\)\\(:?$\\|\\s-\\)" nil t)
      (replace-match "\C-b\\2\C-o" nil nil nil 1))
    (goto-char (point-min))
    (while (re-search-forward "\\(?:^\\|\\s-\\)\\(_\\([^_]+\\)_\\)\\(?:$\\|\\s-\\)" nil t)
      (replace-match "\C-_\\2\C-O" nil nil nil 1))
    (goto-char (point-min))
    (while (re-search-forward "\\(?:^\\|\\s-\\)\\(/\\([^/]+\\)/\\)\\(?:$\\|\\s-\\)" nil t)
      (replace-match "\C-V\\2\C-O" nil nil nil 1))))

(add-hook 'erc-send-pre-hook
          (lambda (string)
              (insert string)
              (setq str (buffer-substring (point-min) (point-max))))))

(add-hook 'erc-insert-modify-hook 'erc-emphasize)

The code transforms the text before sending it, so it will work without changes in the IRC client of the other person, supposed control characters is supported of course. If you want to send/receive text literally, just disable control characters interpretation with C-c C-c in the ERC buffer.