Dude, ik at least 5 for vanilla
Those are not free either, restrictions and other trade offs are made to get them. Are there "no pocket" expenses for Vanilla? Sure but are they technically "free"
nope.
These "free" services will usually limit you..
1. What jars you can use/upload (like you said they are Vanilla only)
2. Uploading rights (if they even grant FTP access at all)
3. Memory (usually in the 256MB-512MB range) and/or other resource limits
4. Hardware/networking may not be optimized and may not even protect against DDoSes
5. Other limitations such as how often you can use them/not 24/7 are most likely present.
These "free" services may make you "pay" by..
1. Personal information harvesting such as filling out surveys (for retail to advertisers and etc)
2. Referral for people to sign up for paid add ons to get basic servicing rights
3.hoping to upsell you to get those add ons or their paid hosting.
At the end of the day your still ends up paying for those so call "free" services. It may not be in dollars and cents but it's some form of trade offs.
If I had to guess on why there are few (if any) "free" hosting for modded servicing. It's most likely because they require much more
quality computing power and networking for just a few people (especially for "Everything but the Kitchen Sink packs". Therefore they cannot make their Return Of Income on these (or at least as easy) as opposed to Vanilla/few plugins Bukkit servers (those can run on crapper CPUs and less RAM).