This application can help you convert numbers between different bases (also known as radixes). It supports all the common bases like binary, octal, decimal, and hexadecimal.
It includes less common bases such as three, four, all the way up to base 36. It also contains specialized ones such as the unary base (composed only of a single character). Another one is Base64, which is a special base for data encoding. Negative bases are supported as well.
There are also some, that are not really bases but are still very useful. It is ASCII (for text encoding) and Roman numerals.