The bookmarklets below are for WordPress websites that do not display a login link for site editors.

If you already have a wp-edit bookmark, it is better to delete it before trying to replace it.

Drag and drop the wp-edit link onto your browser Bookmarks toolbar (Firefox), or Favorites bar (IE), or Bookmarks bar (Chrome). If the bookmarks toolbar is not visible, you may need to enable it using the View options in your browser.
In a WordPress based site, if you have a page open that you want to edit, but you aren't logged on yet, you can click the wp-edit link and WordPress will bring up the login dialog (if necessary), and after the login, will redirect the browser back to the page that was open, but it will be in edit mode.

wp-edit

Drag and drop the wp-admin link onto your bookmarks toolbar for the case where you don't want to edit the currently displayed document but you want to go directly to admin mode on the WordPress based site.

wp-admin

Right-click the wp-login link and select "Bookmark this link" (Firefox), or "Add to Favorites" (IE), or drag and drop the wp-login link to the toolbar. (there is no context menu item labeled Add to Bookmarks in Chrome.)
If you don't have an editable page open, wp-edit won't be able to find a post #. In that case, wp-login can be useful because it opens the WordPress login dialog (only if you have a WordPress based site open).

wp-login

wp-login is supposed to be smart enough that it will not initiate a login when you are already logged in. If you are already logged in, wp-login should redirect you to wp-admin mode. You may decide you don't like or don't use the wp-admin or wp-login bookmarklet. You can delete them. You have the power.