Hexit is a simply designed, easy-to-use calculator that enables you to convert between different number systems on the fly.
Its interface displays whichever value you enter in the decimal, hexadecimal and binary systems, performing the conversion on the fly as you type. Well, you actually have to click on the number buttons; at least I couldn't find any way to use the keyboard and type numbers instead, and I found this pretty annoying.
In all, this is a simple calculator that enables you to perform simple maths operations and logic functions, and hardly uses system resources. It doesn't even require installation.