Social media tools help you prepare content and check how it will look before you post. The Social Media Image Resizer crops and resizes a picture to common social media dimensions, which saves guessing at sizes. The Open Graph Preview shows how a link will appear when shared on Facebook or LinkedIn, so you can fix a missing image or a bad title first.
For writing, the Instagram Caption Generator offers caption ideas based on your photo's topic, and the Hashtag Generator builds a batch of relevant hashtags from your keywords. Treat both as starting points and edit them to fit your own voice, since generated suggestions are generic by nature. The YouTube Thumbnail Downloader saves a video's thumbnail in several resolutions.
Most tools run entirely in your browser. The YouTube Thumbnail Downloader is the exception: it needs to fetch the thumbnail from YouTube, so it uses an outside service. Only download thumbnails you have the right to use, because the image belongs to its creator. It also helps to preview a link before you publish it, since a post with a broken image or wrong title is hard to fix once people have shared it.