You can see the custom version number both in the window title and towards the bottom left
VersionChanger allows modifying the displayed minecraft version with a simple config file.
Any normal text is supported, not just numbers.
Please note that some mods may also read the version and give strange output.
Corrupt minecraft text by translating it
Downloads and updates resource packs automatically
Replaces math methods with incorrect alternatives, causing interesting corruption
Dynamiclly generates a number of empty mods