I execute this code on python 2.4.2 (authentic copy from 
console):
Python 2.4.2 (#1, Oct 26 2005, 14:45:33)
[GCC 2.95.4 20020320 [FreeBSD]] on freebsd4
Type "help", "copyright", "credits" or "license" for more 
information.
>>> a = range(1,10000000)
>>> del a

before i type del - i run top and get (see console output 
below):
16300 sin           2   0   162M   161M poll     0:02 35.76% 
 9.28% python2.4

after del (console below):
16300 sin           2   0   162M   161M poll     0:03  7.18% 
 6.05% python2.4

I tried gc too ... but python didn't free memory. I checked 
this on windows - memory was freed, but interpreter with 
0 defined variables "eat" about 75 Mb!. I think this is bug 
in interpereter core.

some text from dmesg for you:
Copyright (c) 1992-2003 The FreeBSD Project.
Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 
1992, 1993, 1994
        The Regents of the University of California. All rights 
reserved.
FreeBSD 4.8-RELEASE #0: Thu Apr  3 10:53:38 GMT 
2003
    root@freebsd-stable.sentex.ca:/usr/obj/usr/src/sys/
GENERIC
Timecounter "i8254"  frequency 1193182 Hz
CPU: Pentium III/Pentium III Xeon/Celeron (499.15-MHz 
686-class CPU)
  Origin = "GenuineIntel"  Id = 0x673  Stepping = 3
  Features=0x387f9ff<FPU,VME,DE,PSE,TSC,MSR,
PAE,MCE,CX8,SEP,MTRR,PGE,MCA,CMOV,PAT,
PSE36,PN,MMX,FXSR,SSE>
real memory  = 268369920 (262080K bytes)
avail memory = 255901696 (249904K bytes)