We currently have the 80 cm Spitzer telescope in space that covers the same wavelength range, and the 3.5 m Herschel telescope also recently finished its mission at slightly longer wavelengths. So really to make breakthroughs in science to justify the cost of a space mission, we need the full 6.5 m telescope.
There is also SOFIA [1] (2.5m telescope on an airplane), which can do certain far-IR things (mid-IR as well iirc).
For the VLT (8m ground-based telescopes) there is also VISIR [2] which is an mid-IR imager/spectrometer (and uses the same detector chip as one of the JWST instruments iirc).
Oh yeah, those do cover the same wavelength range. The atmosphere of the Earth absorbs a lot of the light and has a strong thermal background though. The sensitivity of JWST is orders of magnitude better than ground/airborne observatories [1], so things that JWST want to do will be basically impossible from the ground.
