Lots of hardware
There is a good number of hardware platforms that do just that without the aid of signatures.
On the software side Panda is probably the leader though you can buy the extra 'Outbreak Prevention Policy (OPP) module from TrendMicro which does basically the same thing for a bit more money.
We haven't had a lot of problems here save a macro viruses from clients so we aren't ready for such a huge investment... yet. It will happen eventually. Spyware seems to be more of an issue than anything virus oriented as of late.
- beads