I know, you're probably surprised and feeling humored. Dictionary.com is so commonplace it can't really be listed as a RUS, right?
I disagree! Sometimes the most commonplace things lose their versatility, because, well, they're so commonplace. That's where this post comes in. Dictionary.com has a couple of very useful stuff under its belt, which haven't enjoyed halfway-near the same popularity as the word definition check-up feature.
There's a Thesaurus, which is actually very good and is perhaps something you've heard of. We're so used to resorting to synonyms our word processors serve us. Not necessarily the right thing to do: lots of similar words have vastly different contextual usages, something your word processor won't tell you.
The site's Encyclopedia will hardly turn up in search engine results, but is very handy. In fact, there are a lot of entries you won't find elsewhere -- even on Wikipedia! If you're studying linguistics, for instance, this is a must-visit for you.
More: Dictionary.com has multilingual dictionaries, widgets, RSS feeds, blogs, toolbars and more! If you're tired of having to go online, grab the toolbar (or even the recommended CleverKeys, which isn't, however, available for Vista). If you think the toolbar is too much, go for the no-nonsense bookmarklet.
To sum up, you've probably frequented Dictionary.com, but do take a closer look. It can help you with your scholarly pursuits (and even help you learn a new language!) with resources that you probably won't find elsewhere. Hop off to Dictionary.com to explore.
For: Very useful definition, synonym and reference lookup features; also has multilingual dictionaries and translators.
Against: Needs to upgrade to Web 2.0; premium content hardly makes sense; we could do with less ads.