There's also the Knight's, IWC, B5, Ergo, Larue, Gear Sector, and Deliberate Dynamics.
I would recommend that you search Google Images for "picatinny hand stop" so that you can see pictures of the items.
You could also self-chop the polymer ones such as the Magpul or even convert a more traditional stubby vertical foregrip or even full-length one (just be sure that it uses horizontal cross-bolts to secure, and not a vertical one like the full-size Daniel Defense), which was very popular in the days before handstops became more popular.