You can check the website points.com. It's legit and provides transfers. When it performs a transfer, it's via transfers authorized by the subject programs (sometimes more than one).

However, when available, transfers are almost always at unfavorable ratios. I think I remember once seeing a transfer at an 85% ratio (i.e. 15% haircut). Some have much larger reductions.

My sense is that transfers from hotel programs to airline programs tend to be the best. Starwood used to have a ratio of 1:1 or sometimes even 1:1.25 for conversion to airlines points. That's gone now of course since there is no more Starwood.

Sidebar: I once used the site because Air Canada had a promotion where you got a certain number of points toward a promotion for each "activity." An activity included a transfer into Air Canada of any number of points. The ratio of USAir (it was about 8 years ago) to Air Canada was something like 3:1 or 5:1, but by converting 50 US points to 10 AC points in 10 transactions I was able to get 10 "activities" and ended up with a very nice bonus from the promotion.