This is one of the oldest and most common file formats, it does have some serious issues but it's flexibility and cross platform compatibility make it a frequent lowest common denominator.
Recently I have been using CSV files with Microsoft Excel and a couple of noteworthy points came to light. First Microsoft Excel cannot cope with numbers longer than 15 digits. Oh and it does not warn you about this, it will just take the first 15 digits starting from the left and drop the rest. Secondly it is hard to get ID numbers with leading zeros into Excel as text, this is true with mobile phone numbers for example but also IDs like 0001, which becomes 1. There are two options, you can write
="01" into your CSV file or
"="01"", which whilst looking odd does actually work.
Files with the extension ".MD" which contain Markdown are becoming increasingly common. GitHub uses them as does Stash from Atlassian. However GitHub openly admit they use their own format of Markdown. Here are some handy hints:
If you visit Babelmark 2 - Compare markdown implementations and try your Markdown, you will see how different engines parse things slightly differently, but this is a helpful resource.