Sadly, for a few years now I've had TDP as one of the main criteria when buying parts for my machines, so there really isn't enough waste heat from my machines to even just keep a room warm in Winter by playing heavy 3D games (the worst machine tops at around 180W with 3D and CPU heavy games - so basically the same heating as a really bright incandescent light bulb - whilst my home server uses about 20W at 100%)
On the other hand what I save in power consumption on my machines can be used on a dedicated heating solution that's ON only when I need it rather than the whole year.