Funny you mention python as, until last month when PyCharm community edition was released, there was no decent and free IDE for python. :)

I can't think of any tools for .net that are really must-haves and that cost money. Maybe resharper but you can survive without it. There are free profilers, decompilers, analysis tools, SCMs, you name it. The only thing you don't get with express is integration with plugins in the IDE. Using .NET with free tools is also easy, connecting to MySQL is just as easy as to MS SQL Server to give an example.