Athlon 64's (including X2's and Opteron).
Pentium D (dual core)
Pentium (But only certain models, look for EM64T support)
Core 2 Duo (but not the older Core Duo)
These are the 64bit processors for Windows PC's. (Although can run other OS's like Linux, Solaris etc.
Additionally, for workstations, non windows environments
UltraSparc
Itanium (Intels 64bit 'non X86 compatible processor)
Sony's Cell processor
PowerPC
There are others, but these are the more 'common' processors, and I've not included any processor which is now out of common use.