vmark.vim

vmark.vim (GitHub: vim-scripts / vmark.vim--Visual-Bookmarking) by Michael Zhou is a visually-oriented bookmarking system where lines can be bookmarked in normal mode.

The foreground and background colours are configurable, so if it doesn't quite work with your colours it's easy to adapt it.

Jacinto

Jacinto (License: MIT) by Alfredo Deza is a JSON formatter and validator. Typing :Jacinto validate will validate the current buffer. Validation works using python -m json.tool, so a working Python installation is required for validation, but the rest of the plugin is VimL.