Internet Explorer engine(HTML - Trident) is needed for a lot of things,
Source. I dont know how up to date and accurate this information is but users should see for themselves.
Some other Trident-based applications include:
AOL Explorer, a web browser
AOL Instant Messenger 6.x, which uses Trident to render conversation and profile windows, and advertisement panels
Avant Browser
Bento Browser (built into Winamp)
EA Link, incompatible with Trident as of Internet Explorer 7 RC2
Google Talk, which uses Trident to render chat windows and profile cards
GreenBrowser, which is also presented at the BrowserChoice.eu page
IE Tab, a Firefox and Google Chrome add-on used to render pages with Trident within the Firefox or Chrome user interface.
Impulse (content delivery), uses Trident to render "Explore" page, as well as several of the "Community" pages
LimeWire, which renders the page 'New@Lime'
Lunascape, developed by Lunascape Corporation
Maxthon, which uses the Trident engine while adding features not built into IE7
MediaBrowser, customized browsers, especially for Nintendo
MenuBox, a web browser
Microsoft Compiled HTML Help
Microsoft Encarta and related products
Microsoft InfoPath
Microsoft Outlook which uses Trident to render HTML Messages (prior to Outlook 2007) and the "Outlook Today" screen
Microsoft Outlook Express, which uses Trident to render HTML Messages
Microsoft Visual InterDev 6 uses Trident in editing mode as visual HTML designer
Microsoft Visual Studio 2002-2005 use Trident in editing mode to provide visual ASP.NET/HTML designer
Microsoft Visual Studio and Visual Basic to render the WebBrowser control
MSN Messenger, which uses it to produce Flash-based "winks" and games, and for all advertisements shown in the advertisement banner
NeoPlanet, a web browser
NetCaptor, a web browser
Netscape Browser (Netscape 8), which used Trident to render web pages in IE mode
Pys, a python Widget set Toolkit. Embedding IWebBrowser2 as an Active-X component and accessing the COM interface, Pyjs uses Trident for the Desktop version, through the python win32 "comtypes" library.
Phoenix Browser, a lightweight web browser
RealNetworks
SiteKiosk, a kiosk browser software
Sleipnir, a web browser
SlimBrowser, a web browser
Skype, software for VoIP that renders HTML data with Trident
TomeRaider, an ebook reader
Tencent Traveler, a web browser
ThreeTeeth Browser, a lightweight web browser
UltraBrowser, a web browser
Valve's Steam client, previous versions of which used Trident to render the "Store", "Update News" and "Community" sections as well as the Steam in-game browser and MOTD screens in Valve games. The Steam client was updated to use WebKit instead of Trident for these features. Then was updated further to use the Chromium Embedded Framework.
WebbIE, a web browser
Windows Live Writer, which uses Trident for its editor
Windows Media Player, which uses Trident to render the "Media Information" pages
360 Secure Browser, a web browser in China