The most common one is low dose naltrexone. I got mine compounded in 1mg tablets, scored so I could start at 0.5mg/day and work up gradually. My insurance covers 1.5 and 4.5mg tablets, so I had to pay a bit for the compounded ones ($68 for 90 1mg tabs - that seemed worth it).

The Long Covid doctor at Mayo recommended several other possible options to talk with my doctors about: modafinil, ketotifen, guanfacine, rapamycin - depending on what the specific issues are. Low dose naltrexone is supposed to help with autonomic issues, as well as fatigue and pain; ketotifen can help with shortness of breath and digestive problems, etc.