Windows: If you're looking to replace Notepad with a lightweight alternative that has some extra bells and whistles, Xint is a capable replacement.
Notepad is ubiquitous but rather underpowered, Xint is a freeware alternative that sports a host of added features to make text editing easier. You can specify the kind of programming language you're working in for syntax based colour-coding, code completion, and expression evaluation. Even if you don't frequently work with code Xint has Twofish-based encryption, active URLs, hot keys and quick formatting macros.
If you check out Xint and you're not sold, make sure to check out our Hive Five on best text editors to take a peek at some powerful Notepad alternatives. If you're not sure how to replace Notepad as your default editor, check out this guide we shared on swapping out Notepad. Have a favourite Notepad-alternative of your own? Let's hear about it in the comments.