TextMate is a lightweight text editor that can be used for writing not only notes but also code. In this respect, the program supports various programming languages. Its interface is quite austere, which, on the one hand, helps you focus on your task, but, on the other, makes it look quite unsophisticated, even when, in this case, appearances are deceptive. There is a toolbar on the bottom from which you can change the language, access settings, and record macros. It is also notable that you can work on multiple documents either on separate windows or different tabs.
However, the ones mentioned are not only features intended to help programmers do their job. For instance, it supports using snippets with placeholders, swapping pieces of code as well as inserting foldable code blocks. Likewise, the tool includes a powerful search function, which lets you jump between matches and preview the intended replacements. You may also find quite convenient the possibility to keep a history of the use of the clipboard. What is more, one of its best features is that it comes with multiple customization bundles intended for specific languages, workflows and markup systems.
All in all, TextMate may be the tool you are looking for if you need to write various types of text. Although this product is developed as opensource, it is not available for free.