There have been some exciting software-isolation-based SASOS attempts, as others have pointed out.
However, a single address space does not necessarily mean that all apps have permissions to all memory and rely on software for isolation. SASOS can also be done in hardware.
Mungi OS was an interesting example of a SASOS using hardware memory protection (developed on SGI MIPS-based workstations back in the day). If you look at table 1 in
http://citeseerx.ist.psu.edu/viewdoc/do ... 1&type=pdf you will see very favourable IPC performance making it all worth it.
The Mungi recipe could be used on x86-64 or any other modern CPU with conventional TLB etc too. (It would be interesting to understand the extent to which L4-style IPC improves etc in such an effort on modern CPUs).
The Mill CPU is an example of hardware which is intrinsically SAS. And an OS we are porting to the Mill is ... Linux! With the Mill's hardware assistance there is nothing stopping Multiple-address-space (MAS) OS being ported to the Mill, with all apps being completely oblivious to the fact that its a SASOS (with no aliasing of virtual memory addresses).