Hot Lingo - description from Softdepia.com:
Hot Lingo is a spell checking plug-in for Microsoft Internet Explorer 5.0 and higher. It is very easy-to-use and powerful and has a rich set of features.
Hot Lingo features built-in 75,000-word English dictionary, that contains both American and British variants. The latest version is easily customized and includes easily configurable user dictionary that contains up to 32,000 user-defined words, www.hotmail.com 'rich text' capabilities, and online meaning lookup. Hot Lingo supports corporate messaging and intranets, features applet support for 2 contemporary most popular remote messaging products Lotus iNotes, and Microsoft Exchange Server Outlook Web Access (OWA) 5.x and 2000.
Hot Lingo is a "Free to try" program. You can download and try it for an evaluation period.