Process Hacker is a tool for viewing and manipulating processes and their threads, modules, memory and handles, and viewing and editing services. It can also search through process memory:

for a sequence of bytes,
using regular expressions,
for strings, or
for heap blocks
It can run programs as another user or as SYSTEM, LOCAL SERVICE or NETWORK SERVICE if you have administator privileges. This is achieved using a helper program which is installed as a system service (similar to PsExec).

It loads symbols from various libraries automatically and uses them when displaying the call stacks of threads (just like Process Explorer). It also has a disassembler derived from OllyDbg's which can be accessed when viewing PE files' exports.

System Requirements
.NET Framework 2.0
Microsoft Windows XP or above
1GB or more of total RAM if you plan on searching or editing memory
2 CPUs (physical or multi-core), if you plan on searching memory