How can I return system information in Python?

Regarding cross-platform: your best bet is probably to write platform-specific code, and then import it conditionally. e.g.

import sys
if sys.platform == 'win32':
  import win32_sysinfo as sysinfo
elif sys.platform == 'darwin':
  import mac_sysinfo as sysinfo
elif 'linux' in sys.platform:
  import linux_sysinfo as sysinfo
#etc

print 'Memory available:', sysinfo.memory_available()

For specific resources, as Anthony points out you can access /proc under linux. For Windows, you could have a poke around at the Microsoft Script Repository. I'm not sure where to get that kind of information on Macs, but I can think of a great website where you could ask :-)

In a Linux environment you could read from the /proc file system.

~$ cat /proc/meminfo
MemTotal:      2076816 kB
MemFree:        130284 kB
Buffers:        192664 kB
Cached:        1482760 kB
SwapCached:          0 kB
Active:         206584 kB
Inactive:      1528608 kB
HighTotal:     1179484 kB
HighFree:       120768 kB
LowTotal:       897332 kB
LowFree:          9516 kB
SwapTotal:     2650684 kB
SwapFree:      2650632 kB
Dirty:              64 kB
Writeback:          12 kB
AnonPages:       59668 kB
Mapped:          22008 kB
Slab:           200744 kB
PageTables:       1220 kB
NFS_Unstable:        0 kB
Bounce:              0 kB
CommitLimit:   3689092 kB
Committed_AS:   263892 kB
VmallocTotal:   114680 kB
VmallocUsed:      3604 kB
VmallocChunk:   110752 kB

psutil should provide what you need:

[...] cross-platform library for retrieving information on running processes and system utilization (CPU, memory, disks, network) [...]

[...] supports Linux, Windows, OSX, FreeBSD and Sun Solaris, both 32-bit and 64-bit architectures [...]