Empty HTML tags are tags that do not need to be closed when one is creating a HTML document. Some people consider the paragraph tag to be an empty tag, but they are merely non-compliant, dirty-coding fools. The World Wide Web Consortium on the other hand know what they are talking about and try to keep things under control with "compliance", "standards" and other such buzzwords(!)

Anyhoo, here is the official list of empty HTML tags for the 4.01 flava:

AREA
Client-side image map area.

BASE
Document base URL.

BASEFONT
Base font size.

BR
Forced line break.

COL
Table column.

FRAME
Subwindow.

HR
Horizontal rule.

IMG
Embedded image.

INPUT
Form control.

ISINDEX
Single line prompt.

LINK
A media independent link.

META
Generic metainformation.

PARAM
Named property value.

Tech note 1: Although I have written the tag names in uppercase, HTML 4.01 tags are now case insensitive. This is an effort by W3C to bring HTML more in line with XML and persumably to give web developers like myself less headaches.

Tech note 2: As pointed out by Millennium these tags do need to be closed when writing XHTML. See below for further info.

Further info on the joys of HTML 4.01 and can be found at http://www.w3.org.