CSS, SCSS, & HTML
CSSLint helps you find problems with your CSS code. It does basic syntax checking as well as applying a set of rules to the code that look for problematic patterns or signs of inefficiency. The rules are all pluggable, so you can easily write your own or omit ones you don't want.
Checkstyle is a development tool to help programmers write Java code that adheres to a coding standard. By default it supports the Google Java Style Guide and Sun Code Conventions, but is highly configurable. It can be invoked with an ANT task and a command line program.
RuboCop is a Ruby static code analyzer. Out of the box, it will enforce many of the guidelines outlined in the community Ruby Style Guide.
C and C++
HLint is a tool for suggesting possible improvements to Haskell code. These suggestions include ideas such as using alternative functions, simplifying code and spotting redundancies.
Other Useful Linters
ShellCheck, a static analysis tool for shell scripts, automatically detects problems with sh/bash scripts and commands.