Some plugins require the user to insert some codes into the theme for it to function properly. Usually the codes are not long - most often it's a one-liner.
While you can definitely insert those codes into the theme without touching the theme's core files by using the action hooks and filters, it's not easy to achieve if you're not familiar with PHP and WordPress action hooks and filters system.
That is why I am now asking for suggestions on which popular plugins you use that require you to insert codes into the theme, so that I can include native support for those plugins.
For maximum benefits, please limit your plugins suggestions to the ones that conform to these requirements:
1. Plugin must be popular (eg. more than 100 000 downloads)
2. Plugin requires some codes to be inserted into the theme, or
3. Plugin looks ugly/out-of-place when used together with the theme
So here's your chance. Go on and suggest!