To use an online dictionary, you need to be able to type the word you want to look up into a text box on a web page.  If this word is in Hebrew, you will need to have Hebrew support for your computer.

Morfix: This is by far the best online Hebrew-English/English-Hebrew dictionary we have found.  Just type your word, in Hebrew or in English, into the text field, and click on the large blue button.  The web page, however, is written with some Microsoft specific features.  On a Windows PC with MSIE 5 or later, you can use the Hebrew keypad (leftmost link in small blue letters) to enter Hebrew characters.  Or, if you have Hebrew support, switch to the Hebrew keyboard and type your word directly in the search box.  All of the possible vowelizations and roots for your word will be displayed.  On Mac OS X 10.2 or later, with the Safari browser, you can enable the system Hebrew keyboard to type in a Hebrew word to look up.  The results page displays acceptably, although there are some issues with the alignment of the vowels (e.g. in "aniyah" -- alef_nun_yod_hay -- the word for "ship"). 

Zooloo Hebrew to English Dictionary and Zooloo English to Hebrew Dictionary: This works on a Mac with Netscape 6 or later, with Hebrew support, as well as on a Windows PC (MSIE 5 or Netscape 6 or later), with Hebrew support. 

My Hebrew Dictionary: An extensive word list, organized by subject matter.  No Hebrew support of any type is needed.