I've been finding the spell checking feature of Mathematica surprisingly helpful at identifying typos in variable names. It makes me wonder why more language processors (compilers and interpreters) don't make use of this.
The feature works like this. Each new identifier reference is checked for similarity to an already known symbol. If it is similar but not the same, a warning is generated, as follows:
In := rootDir = "/tmp/foo";
setup = rootdir <> "/index.html"
Possible spelling error: new symbol name "rootdir" is
similar to existing symbol "rootDir". More...
(For more info see the documentation for General::spell1 and its examples).
The rootdir is flagged as similar to rootDir. In Mathematica this is important because variables don't need to be declared before use, which makes sense for supporting symbolic algebra. In a language like Java where variables must be declared, the compiler would catch rootdir as undeclared, but would leave it to you to figure out why. With a spellchecker to catch the similarity you could get a possibly helpful hint.
The chief drawback I see is the noise you get from false positives, when the spell checker finds a similarity between a symbol you just wrote with an existing symbol, and you know full well that they're different and it's OK. It's a little like the Dick Van Dyke show exchange when Laura asks Rob "Do you or do you not like reminders?" and Rob replies "Only when I forget." Mathematica let you disable the spell check feature, so you don't see the warning every time you load a bit of code with a spelling similarity.
It would be cool to combine this spellchecking feature with the "Quick fix" feature in Eclipse. You make a typo in a variable name, it gets a compile error perhaps because it's not declared in its current form, and the compiler additionally warns "similar to symbol X". The quick fix would be "change to X".