How big of a group is "many"? As mentioned already, Webmin is good if it does what you want - you can run arbitrary commands and do some common admin tasks (like synchronizing local users) using https. So, you have less overhead than the ssh method. Webmin's cluster tools are good for "tens" of machines; I've never tried it on larger groups. There's also cfengine, which was mentioned - it can be used for small to extremely large groups of machines (as in, thousands), because it can do tiered management as needed (one master, then some sub-masters, etc). I'm currently using this to manage a network which is about 3500 Unix machines of different flavors. As the other poster said, it's a pain to initially learn, but works very well.
If your systems are homegeneous, or at least fairly homogeneous groups, and you don't have a whole lot (like, under a few hundred), there are several good cluster management toolsets out there. The Oscar project has some admin tools already assembled for use in managing a cluster: http://svn.oscar.openclustergroup.org/trac/oscar, and there are other similar projects whose names escape me (I'll probably remember as soon as I post).
In simpler terms, there are a few paralell ssh tools discussed at the Linux Journal: http://www.linux.com/archive/feature/151340. Like the other ssh-based cluster tools though, you'll still start running into problems when you try to open too many concurrent ssh connections. Depending on your hardware, my esperience is that you'll probably want to keep paralelism below 20-30 simultaneous ssh links.
Basically, there are a whole lot of pre-made solutions for this, and there are just as many home-grown solutions. Look around freshmeat.net and google and you'll find several. Or roll your own; it's not a particularly difficult challenge to solve acceptably if you're just doing 10-20 machines... :)