No, if you go here (http://www.minecraft.net/download.jsp) and download one of the files under "Multiplayer Beta Server Software" heading, you can host a multiplayer server from your PC.
However, it is difficult to maintain a server without investing in a dedicated server.
Note that the multiplayer server software is still in deveopment, and from personal experience it can be a bit finnikey. I would really only recomend hosting it off of your PC if you're planning on playing with friends only.