Gurunet
http://www.gurunet.com
= must have
Provides fast reference, translation, and searches.

GuruNet is a free one-click information service that works whenever you're online. It automatically analyzes the pointed-to text in context and pops up a simple window without linking or leaving your document; you don't even have to select the word.

The GuruNet window opens automatically, displaying information about the specific word, and you can choose from various category tabs to view different kinds of information about the word. Depending on the word you clicked, you'll get reference (e.g. dictionary, encyclopedia) and/or real-time information (e.g. news, sports, stock quotes).

GuruNet works in any PC application, such as e-mail, MS-Office, PIMs and any browser, and it is relatively compact: only 700 kilobytes. Once loaded, it resides quietly in the Windows system tray until needed. It works only when the user has an open connection to the Internet, so it is most useful to people who are connected through an office network, or a continous connection. People with dial up modems may encounter problems when trying to use Gurunet.

The Gurunet software is activated by holding down the ALT key and clicking on any word, which will open a new window containing a dictionary definition of the word. To the left of the definition are choices that include, depending on the context, a dictionary definition, an encyclopedia entry, a list of hyperlinks to relevant Web sites, a business profile or biography, a stock quote, or other useful factoids and info nuggets. In most cases, there is no need to type search commands.

Like many of the other tools viewed here, Gurunet is able to search in context. For example, clicking on the word "Clinton" in a sentence that mentions Hillary Rodham Clinton calls up a biographical mention of the First Lady, not her husband, Bill, nor George Clinton of the P-Funk All-Stars. It scans a half-dozen words on either side of the highlighted word for clues to the context, and, more often than not, it hits the mark.

Clicking on "AT&T" in an E-mail message calls up the company profile, its stock ticker symbol, stock chart and current stock price, and a selection of recent news headlines pointing to news stories mentioning AT&T.

Gurunet is still a work in progress. Its success will depend on the addition of information databases, which could include more reference works, E-commerce partners, product catalogues, news services, and so on.

PCMagazine has elected Gurunet one of the best tools of 1999.
You can read their review at: www.zdnet.com/products/stories/reviews/ 0,4161,2402299,00.html
I personally like this tool the best out of the four reviewed. To me it is the one of the most effective and has the best designed interface. I truly find it useful.